diff --git a/code/controllers/subsystem/tgui.dm b/code/controllers/subsystem/tgui.dm
index 58bc28fa2f..a5526d2c03 100644
--- a/code/controllers/subsystem/tgui.dm
+++ b/code/controllers/subsystem/tgui.dm
@@ -1,3 +1,12 @@
+/**
+ * tgui subsystem
+ *
+ * Contains all tgui state and subsystem code.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
+ */
+
 SUBSYSTEM_DEF(tgui)
 	name = "tgui"
 	wait = 9
@@ -5,33 +14,338 @@ SUBSYSTEM_DEF(tgui)
 	priority = FIRE_PRIORITY_TGUI
 	runlevels = RUNLEVEL_LOBBY | RUNLEVELS_DEFAULT
 
-	var/list/currentrun = list()
-	var/list/open_uis = list() // A list of open UIs, grouped by src_object and ui_key.
-	var/list/processing_uis = list() // A list of processing UIs, ungrouped.
-	var/basehtml // The HTML base used for all UIs.
+	/// A list of UIs scheduled to process
+	var/list/current_run = list()
+	/// A list of open UIs
+	var/list/open_uis = list()
+	/// A list of open UIs, grouped by src_object.
+	var/list/open_uis_by_src = list()
+	/// The HTML base used for all UIs.
+	var/basehtml
 
 /datum/controller/subsystem/tgui/PreInit()
-		basehtml = file2text('tgui/packages/tgui/public/tgui.html')
+	basehtml = file2text('tgui/packages/tgui/public/tgui.html')
 
 /datum/controller/subsystem/tgui/Shutdown()
 	close_all_uis()
 
 /datum/controller/subsystem/tgui/stat_entry()
-	..("P:[processing_uis.len]")
+	..("P:[open_uis.len]")
 
 /datum/controller/subsystem/tgui/fire(resumed = 0)
-	if (!resumed)
-		src.currentrun = processing_uis.Copy()
-	//cache for sanic speed (lists are references anyways)
-	var/list/currentrun = src.currentrun
-
-	while(currentrun.len)
-		var/datum/tgui/ui = currentrun[currentrun.len]
-		currentrun.len--
+	if(!resumed)
+		src.current_run = open_uis.Copy()
+	// Cache for sanic speed (lists are references anyways)
+	var/list/current_run = src.current_run
+	while(current_run.len)
+		var/datum/tgui/ui = current_run[current_run.len]
+		current_run.len--
+		// TODO: Move user/src_object check to process()
 		if(ui && ui.user && ui.src_object)
 			ui.process()
 		else
-			processing_uis.Remove(ui)
-		if (MC_TICK_CHECK)
+			open_uis.Remove(ui)
+		if(MC_TICK_CHECK)
 			return
 
+/**
+ * public
+ *
+ * Requests a usable tgui window from the pool.
+ * Returns null if pool was exhausted.
+ *
+ * required user mob
+ * return datum/tgui
+ */
+/datum/controller/subsystem/tgui/proc/request_pooled_window(mob/user)
+	if(!user.client)
+		return null
+	var/list/windows = user.client.tgui_windows
+	var/window_id
+	var/datum/tgui_window/window
+	var/window_found = FALSE
+	// Find a usable window
+	for(var/i in 1 to TGUI_WINDOW_HARD_LIMIT)
+		window_id = TGUI_WINDOW_ID(i)
+		window = windows[window_id]
+		// As we are looping, create missing window datums
+		if(!window)
+			window = new(user.client, window_id, pooled = TRUE)
+		// Skip windows with acquired locks
+		if(window.locked)
+			continue
+		if(window.status == TGUI_WINDOW_READY)
+			return window
+		if(window.status == TGUI_WINDOW_CLOSED)
+			window.status = TGUI_WINDOW_LOADING
+			window_found = TRUE
+			break
+	if(!window_found)
+		log_tgui(user, "Error: Pool exhausted")
+		return null
+	return window
+
+/**
+ * public
+ *
+ * Force closes all tgui windows.
+ *
+ * required user mob
+ */
+/datum/controller/subsystem/tgui/proc/force_close_all_windows(mob/user)
+	log_tgui(user, "force_close_all_windows")
+	if(user.client)
+		user.client.tgui_windows = list()
+		for(var/i in 1 to TGUI_WINDOW_HARD_LIMIT)
+			var/window_id = TGUI_WINDOW_ID(i)
+			user << browse(null, "window=[window_id]")
+
+/**
+ * public
+ *
+ * Force closes the tgui window by window_id.
+ *
+ * required user mob
+ * required window_id string
+ */
+/datum/controller/subsystem/tgui/proc/force_close_window(mob/user, window_id)
+	log_tgui(user, "force_close_window")
+	// Close all tgui datums based on window_id.
+	for(var/datum/tgui/ui in user.tgui_open_uis)
+		if(ui.window && ui.window.id == window_id)
+			ui.close(can_be_suspended = FALSE)
+	// Unset machine just to be sure.
+	user.unset_machine()
+	// Close window directly just to be sure.
+	user << browse(null, "window=[window_id]")
+
+/**
+ * public
+ *
+ * Try to find an instance of a UI, and push an update to it.
+ *
+ * required user mob The mob who opened/is using the UI.
+ * required src_object datum The object/datum which owns the UI.
+ * optional ui datum/tgui The UI to be updated, if it exists.
+ * optional force_open bool If the UI should be re-opened instead of updated.
+ *
+ * return datum/tgui The found UI.
+ */
+/datum/controller/subsystem/tgui/proc/try_update_ui(
+		mob/user,
+		datum/src_object,
+		datum/tgui/ui)
+	// Look up a UI if it wasn't passed
+	if(isnull(ui))
+		ui = get_open_ui(user, src_object)
+	// Couldn't find a UI.
+	if(isnull(ui))
+		return null
+	ui.process_status()
+	// UI ended up with the closed status
+	// or is actively trying to close itself.
+	// FIXME: Doesn't actually fix the paper bug.
+	if(ui.status <= UI_CLOSE)
+		ui.close()
+		return null
+	ui.send_update()
+	return ui
+
+/**
+ * public
+ *
+ * Get a open UI given a user and src_object.
+ *
+ * required user mob The mob who opened/is using the UI.
+ * required src_object datum The object/datum which owns the UI.
+ *
+ * return datum/tgui The found UI.
+ */
+/datum/controller/subsystem/tgui/proc/get_open_ui(mob/user, datum/src_object)
+	var/key = "[REF(src_object)]"
+	// No UIs opened for this src_object
+	if(isnull(open_uis_by_src[key]) || !istype(open_uis_by_src[key], /list))
+		return null
+	for(var/datum/tgui/ui in open_uis_by_src[key])
+		// Make sure we have the right user
+		if(ui.user == user)
+			return ui
+	return null
+
+/**
+ * public
+ *
+ * Update all UIs attached to src_object.
+ *
+ * required src_object datum The object/datum which owns the UIs.
+ *
+ * return int The number of UIs updated.
+ */
+/datum/controller/subsystem/tgui/proc/update_uis(datum/src_object)
+	var/count = 0
+	var/key = "[REF(src_object)]"
+	// No UIs opened for this src_object
+	if(isnull(open_uis_by_src[key]) || !istype(open_uis_by_src[key], /list))
+		return count
+	for(var/datum/tgui/ui in open_uis_by_src[key])
+		// Check if UI is valid.
+		if(ui && ui.src_object && ui.user && ui.src_object.ui_host(ui.user))
+			ui.process(force = 1)
+			count++
+	return count
+
+/**
+ * public
+ *
+ * Close all UIs attached to src_object.
+ *
+ * required src_object datum The object/datum which owns the UIs.
+ *
+ * return int The number of UIs closed.
+ */
+/datum/controller/subsystem/tgui/proc/close_uis(datum/src_object)
+	var/count = 0
+	var/key = "[REF(src_object)]"
+	// No UIs opened for this src_object
+	if(isnull(open_uis_by_src[key]) || !istype(open_uis_by_src[key], /list))
+		return count
+	for(var/datum/tgui/ui in open_uis_by_src[key])
+		// Check if UI is valid.
+		if(ui && ui.src_object && ui.user && ui.src_object.ui_host(ui.user))
+			ui.close()
+			count++
+	return count
+
+/**
+ * public
+ *
+ * Close all UIs regardless of their attachment to src_object.
+ *
+ * return int The number of UIs closed.
+ */
+/datum/controller/subsystem/tgui/proc/close_all_uis()
+	var/count = 0
+	for(var/key in open_uis_by_src)
+		for(var/datum/tgui/ui in open_uis_by_src[key])
+			// Check if UI is valid.
+			if(ui && ui.src_object && ui.user && ui.src_object.ui_host(ui.user))
+				ui.close()
+				count++
+	return count
+
+/**
+ * public
+ *
+ * Update all UIs belonging to a user.
+ *
+ * required user mob The mob who opened/is using the UI.
+ * optional src_object datum If provided, only update UIs belonging this src_object.
+ *
+ * return int The number of UIs updated.
+ */
+/datum/controller/subsystem/tgui/proc/update_user_uis(mob/user, datum/src_object)
+	var/count = 0
+	if(length(user?.tgui_open_uis) == 0)
+		return count
+	for(var/datum/tgui/ui in user.tgui_open_uis)
+		if(isnull(src_object) || ui.src_object == src_object)
+			ui.process(force = 1)
+			count++
+	return count
+
+/**
+ * public
+ *
+ * Close all UIs belonging to a user.
+ *
+ * required user mob The mob who opened/is using the UI.
+ * optional src_object datum If provided, only close UIs belonging this src_object.
+ *
+ * return int The number of UIs closed.
+ */
+/datum/controller/subsystem/tgui/proc/close_user_uis(mob/user, datum/src_object)
+	var/count = 0
+	if(length(user?.tgui_open_uis) == 0)
+		return count
+	for(var/datum/tgui/ui in user.tgui_open_uis)
+		if(isnull(src_object) || ui.src_object == src_object)
+			ui.close()
+			count++
+	return count
+
+/**
+ * private
+ *
+ * Add a UI to the list of open UIs.
+ *
+ * required ui datum/tgui The UI to be added.
+ */
+/datum/controller/subsystem/tgui/proc/on_open(datum/tgui/ui)
+	var/key = "[REF(ui.src_object)]"
+	if(isnull(open_uis_by_src[key]) || !istype(open_uis_by_src[key], /list))
+		open_uis_by_src[key] = list()
+	ui.user.tgui_open_uis |= ui
+	var/list/uis = open_uis_by_src[key]
+	uis |= ui
+	open_uis |= ui
+
+/**
+ * private
+ *
+ * Remove a UI from the list of open UIs.
+ *
+ * required ui datum/tgui The UI to be removed.
+ *
+ * return bool If the UI was removed or not.
+ */
+/datum/controller/subsystem/tgui/proc/on_close(datum/tgui/ui)
+	var/key = "[REF(ui.src_object)]"
+	if(isnull(open_uis_by_src[key]) || !istype(open_uis_by_src[key], /list))
+		return FALSE
+	// Remove it from the list of processing UIs.
+	open_uis.Remove(ui)
+	// If the user exists, remove it from them too.
+	if(ui.user)
+		ui.user.tgui_open_uis.Remove(ui)
+	var/list/uis = open_uis_by_src[key]
+	uis.Remove(ui)
+	if(length(uis) == 0)
+		open_uis_by_src.Remove(key)
+	return TRUE
+
+/**
+ * private
+ *
+ * Handle client logout, by closing all their UIs.
+ *
+ * required user mob The mob which logged out.
+ *
+ * return int The number of UIs closed.
+ */
+/datum/controller/subsystem/tgui/proc/on_logout(mob/user)
+	close_user_uis(user)
+
+/**
+ * private
+ *
+ * Handle clients switching mobs, by transferring their UIs.
+ *
+ * required user source The client's original mob.
+ * required user target The client's new mob.
+ *
+ * return bool If the UIs were transferred.
+ */
+/datum/controller/subsystem/tgui/proc/on_transfer(mob/source, mob/target)
+	// The old mob had no open UIs.
+	if(length(source?.tgui_open_uis) == 0)
+		return FALSE
+	if(isnull(target.tgui_open_uis) || !istype(target.tgui_open_uis, /list))
+		target.tgui_open_uis = list()
+	// Transfer all the UIs.
+	for(var/datum/tgui/ui in source.tgui_open_uis)
+		// Inform the UIs of their new owner.
+		ui.user = target
+		target.tgui_open_uis.Add(ui)
+	// Clear the old list.
+	source.tgui_open_uis.Cut()
+	return TRUE
diff --git a/code/modules/tgui/external.dm b/code/modules/tgui/external.dm
index 5de54c439c..46b324e151 100644
--- a/code/modules/tgui/external.dm
+++ b/code/modules/tgui/external.dm
@@ -1,7 +1,8 @@
 /**
- * tgui external
+ * External tgui definitions, such as src_object APIs.
  *
- * Contains all external tgui declarations.
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 /**
@@ -11,13 +12,9 @@
  * If this proc is not implemented properly, the UI will not update correctly.
  *
  * required user mob The mob who opened/is using the UI.
- * optional ui_key string The ui_key of the UI.
  * optional ui datum/tgui The UI to be updated, if it exists.
- * optional force_open bool If the UI should be re-opened instead of updated.
- * optional master_ui datum/tgui The parent UI.
- * optional state datum/ui_state The state used to determine status.
  */
-/datum/proc/ui_interact(mob/user, ui_key = "main", datum/tgui/ui = null, force_open = FALSE, datum/tgui/master_ui = null, datum/ui_state/state = GLOB.default_state)
+/datum/proc/ui_interact(mob/user, datum/tgui/ui)
 	return FALSE // Not implemented.
 
 /**
@@ -37,10 +34,11 @@
  * public
  *
  * Static Data to be sent to the UI.
- * Static data differs from normal data in that it's large data that should be sent infrequently
- * This is implemented optionally for heavy uis that would be sending a lot of redundant data
- * frequently.
- * Gets squished into one object on the frontend side, but the static part is cached.
+ *
+ * Static data differs from normal data in that it's large data that should be
+ * sent infrequently. This is implemented optionally for heavy uis that would
+ * be sending a lot of redundant data frequently. Gets squished into one
+ * object on the frontend side, but the static part is cached.
  *
  * required user mob The mob interacting with the UI.
  *
@@ -52,18 +50,17 @@
 /**
  * public
  *
- * Forces an update on static data. Should be done manually whenever something happens to change static data.
+ * Forces an update on static data. Should be done manually whenever something
+ * happens to change static data.
  *
  * required user the mob currently interacting with the ui
  * optional ui ui to be updated
- * optional ui_key ui key of ui to be updated
  */
-/datum/proc/update_static_data(mob/user, datum/tgui/ui, ui_key = "main")
-	ui = SStgui.try_update_ui(user, src, ui_key, ui)
-	// If there was no ui to update, there's no static data to update either.
+/datum/proc/update_static_data(mob/user, datum/tgui/ui)
 	if(!ui)
-		return
-	ui.push_data(null, ui_static_data(), TRUE)
+		ui = SStgui.get_open_ui(user, src)
+	if(ui)
+		ui.send_full_update()
 
 /**
  * public
@@ -85,17 +82,12 @@
  * public
  *
  * Called on an object when a tgui object is being created, allowing you to
- * customise the html
- * For example: inserting a custom stylesheet that you need in the head
+ * push various assets to tgui, for examples spritesheets.
  *
- * For this purpose, some tags are available in the html, to be parsed out
- ^ with replacetext
- * (customheadhtml) - Additions to the head tag
- *
- * required html the html base text
+ * return list List of asset datums or file paths.
  */
-/datum/proc/ui_base_html(html)
-	return html
+/datum/proc/ui_assets(mob/user)
+	return list()
 
 /**
  * private
@@ -107,6 +99,15 @@
 /datum/proc/ui_host(mob/user)
 	return src // Default src.
 
+/**
+ * private
+ *
+ * The UI's state controller to be used for created uis
+ * This is a proc over a var for memory reasons
+ */
+/datum/proc/ui_state(mob/user)
+	return GLOB.default_state
+
 /**
  * global
  *
@@ -118,9 +119,17 @@
 /**
  * global
  *
- * Used to track UIs for a mob.
+ * Tracks open UIs for a user.
  */
-/mob/var/list/open_uis = list()
+/mob/var/list/tgui_open_uis = list()
+
+/**
+ * global
+ *
+ * Tracks open windows for a user.
+ */
+/client/var/list/tgui_windows = list()
+
 /**
  * public
  *
@@ -137,17 +146,43 @@
  *
  * required uiref ref The UI that was closed.
  */
-/client/verb/uiclose(ref as text)
+/client/verb/uiclose(window_id as text)
 	// Name the verb, and hide it from the user panel.
 	set name = "uiclose"
-	set hidden = 1
+	set hidden = TRUE
+	var/mob/user = src && src.mob
+	if(!user)
+		return
+	// Close all tgui datums based on window_id.
+	SStgui.force_close_window(user, window_id)
 
-	// Get the UI based on the ref.
-	var/datum/tgui/ui = locate(ref)
-
-	// If we found the UI, close it.
-	if(istype(ui))
-		ui.close()
-		// Unset machine just to be sure.
-		if(src && src.mob)
-			src.mob.unset_machine()
+/**
+ * Middleware for /client/Topic.
+ *
+ * return bool Whether the topic is passed (TRUE), or cancelled (FALSE).
+ */
+/proc/tgui_Topic(href_list)
+	// Skip non-tgui topics
+	if(!href_list["tgui"])
+		return TRUE
+	var/type = href_list["type"]
+	// Unconditionally collect tgui logs
+	if(type == "log")
+		log_tgui(usr, href_list["message"])
+	// Locate window
+	var/window_id = href_list["window_id"]
+	var/datum/tgui_window/window
+	if(window_id)
+		window = usr.client.tgui_windows[window_id]
+		if(!window)
+			log_tgui(usr, "Error: Couldn't find the window datum, force closing.")
+			SStgui.force_close_window(usr, window_id)
+			return FALSE
+	// Decode payload
+	var/payload
+	if(href_list["payload"])
+		payload = json_decode(href_list["payload"])
+	// Pass message to window
+	if(window)
+		window.on_message(type, payload, href_list)
+	return FALSE
diff --git a/code/modules/tgui/states.dm b/code/modules/tgui/states.dm
index e626b98815..383cce7a78 100644
--- a/code/modules/tgui/states.dm
+++ b/code/modules/tgui/states.dm
@@ -1,7 +1,9 @@
 /**
- * tgui states
+ * Base state and helpers for states. Just does some sanity checks,
+ * implement a proper state for in-depth checks.
  *
- * Base state and helpers for states. Just does some sanity checks, implement a state for in-depth checks.
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 /**
@@ -22,13 +24,14 @@
 
 	if(isobserver(user))
 		// If they turn on ghost AI control, admins can always interact.
-		if(IsAdminGhost(user))
+		if(isAdminGhostAI(user)) //IsAdminGhost
 			. = max(., UI_INTERACTIVE)
 
 		// Regular ghosts can always at least view if in range.
-		var/clientviewlist = getviewsize(user.client.view)
-		if(get_dist(src_object, user) < max(clientviewlist[1],clientviewlist[2]))
-			. = max(., UI_UPDATE)
+		if(user.client)
+			var/clientviewlist = getviewsize(user.client.view)
+			if(get_dist(src_object, user) < max(clientviewlist[1], clientviewlist[2]))
+				. = max(., UI_UPDATE)
 
 	// Check if the state allows interaction
 	var/result = state.can_use_topic(src_object, user)
@@ -46,7 +49,8 @@
  * return UI_state The state of the UI.
  */
 /datum/ui_state/proc/can_use_topic(src_object, mob/user)
-	return UI_CLOSE // Don't allow interaction by default.
+	// Don't allow interaction by default.
+	return UI_CLOSE
 
 /**
  * public
@@ -56,21 +60,31 @@
  * return UI_state The state of the UI.
  */
 /mob/proc/shared_ui_interaction(src_object)
-	if(!client) // Close UIs if mindless.
+	// Close UIs if mindless.
+	if(!client)
 		return UI_CLOSE
-	else if(stat) // Disable UIs if unconcious.
+	// Disable UIs if unconcious.
+	else if(stat)
 		return UI_DISABLED
-	else if(incapacitated() || lying) // Update UIs if incapicitated but concious.
+	// Update UIs if incapicitated but concious.
+	else if(incapacitated())
 		return UI_UPDATE
 	return UI_INTERACTIVE
 
+/mob/living/shared_ui_interaction(src_object)
+	. = ..()
+	if(!(mobility_flags & MOBILITY_UI) && . == UI_INTERACTIVE)
+		return UI_UPDATE
+
 /mob/living/silicon/ai/shared_ui_interaction(src_object)
-	if(lacks_power()) // Disable UIs if the AI is unpowered.
+	// Disable UIs if the AI is unpowered.
+	if(lacks_power())
 		return UI_DISABLED
 	return ..()
 
 /mob/living/silicon/robot/shared_ui_interaction(src_object)
-	if(!cell || cell.charge <= 0 || locked_down) // Disable UIs if the Borg is unpowered or locked.
+	// Disable UIs if the Borg is unpowered or locked.
+	if(!cell || cell.charge <= 0 || lockcharge)
 		return UI_DISABLED
 	return ..()
 
@@ -87,7 +101,8 @@
  * return UI_state The state of the UI.
  */
 /atom/proc/contents_ui_distance(src_object, mob/living/user)
-	return user.shared_living_ui_distance(src_object) // Just call this mob's check.
+	// Just call this mob's check.
+	return user.shared_living_ui_distance(src_object)
 
 /**
  * public
@@ -99,17 +114,21 @@
  * return UI_state The state of the UI.
  */
 /mob/living/proc/shared_living_ui_distance(atom/movable/src_object, viewcheck = TRUE)
-	if(viewcheck && !(src_object in view(src))) // If the object is obscured, close it.
+	// If the object is obscured, close it.
+	if(viewcheck && !(src_object in view(src)))
 		return UI_CLOSE
-
 	var/dist = get_dist(src_object, src)
-	if(dist <= 1 || src_object.hasSiliconAccessInArea(src)) // Open and interact if 1-0 tiles away.
+	// Open and interact if 1-0 tiles away.
+	if(dist <= 1)
 		return UI_INTERACTIVE
-	else if(dist <= 2) // View only if 2-3 tiles away.
+	// View only if 2-3 tiles away.
+	else if(dist <= 2)
 		return UI_UPDATE
-	else if(dist <= 5) // Disable if 5 tiles away.
+	// Disable if 5 tiles away.
+	else if(dist <= 5)
 		return UI_DISABLED
-	return UI_CLOSE // Otherwise, we got nothing.
+	// Otherwise, we got nothing.
+	return UI_CLOSE
 
 /mob/living/carbon/human/shared_living_ui_distance(atom/movable/src_object, viewcheck = TRUE)
 	if(dna.check_mutation(TK) && tkMaxRangeCheck(src, src_object))
diff --git a/code/modules/tgui/states/admin.dm b/code/modules/tgui/states/admin.dm
index 61fc373118..227a294078 100644
--- a/code/modules/tgui/states/admin.dm
+++ b/code/modules/tgui/states/admin.dm
@@ -2,6 +2,9 @@
  * tgui state: admin_state
  *
  * Checks that the user is an admin, end-of-story.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(admin_state, /datum/ui_state/admin_state, new)
diff --git a/code/modules/tgui/states/always.dm b/code/modules/tgui/states/always.dm
index a741e2e3d4..210f0896a2 100644
--- a/code/modules/tgui/states/always.dm
+++ b/code/modules/tgui/states/always.dm
@@ -2,6 +2,9 @@
  * tgui state: always_state
  *
  * Always grants the user UI_INTERACTIVE. Period.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(always_state, /datum/ui_state/always_state, new)
diff --git a/code/modules/tgui/states/conscious.dm b/code/modules/tgui/states/conscious.dm
index 4e2793d130..670ca7c07e 100644
--- a/code/modules/tgui/states/conscious.dm
+++ b/code/modules/tgui/states/conscious.dm
@@ -2,6 +2,9 @@
  * tgui state: conscious_state
  *
  * Only checks if the user is conscious.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(conscious_state, /datum/ui_state/conscious_state, new)
diff --git a/code/modules/tgui/states/contained.dm b/code/modules/tgui/states/contained.dm
index f02424d01e..1eb8edba25 100644
--- a/code/modules/tgui/states/contained.dm
+++ b/code/modules/tgui/states/contained.dm
@@ -2,6 +2,9 @@
  * tgui state: contained_state
  *
  * Checks that the user is inside the src_object.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(contained_state, /datum/ui_state/contained_state, new)
diff --git a/code/modules/tgui/states/deep_inventory.dm b/code/modules/tgui/states/deep_inventory.dm
index 43758cbab1..a2b9276a59 100644
--- a/code/modules/tgui/states/deep_inventory.dm
+++ b/code/modules/tgui/states/deep_inventory.dm
@@ -1,7 +1,11 @@
 /**
  * tgui state: deep_inventory_state
  *
- * Checks that the src_object is in the user's deep (backpack, box, toolbox, etc) inventory.
+ * Checks that the src_object is in the user's deep
+ * (backpack, box, toolbox, etc) inventory.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(deep_inventory_state, /datum/ui_state/deep_inventory_state, new)
diff --git a/code/modules/tgui/states/default.dm b/code/modules/tgui/states/default.dm
index 6bb159640e..367e57beff 100644
--- a/code/modules/tgui/states/default.dm
+++ b/code/modules/tgui/states/default.dm
@@ -1,7 +1,11 @@
 /**
  * tgui state: default_state
  *
- * Checks a number of things -- mostly physical distance for humans and view for robots.
+ * Checks a number of things -- mostly physical distance for humans
+ * and view for robots.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(default_state, /datum/ui_state/default, new)
diff --git a/code/modules/tgui/states/default_contained.dm b/code/modules/tgui/states/default_contained.dm
deleted file mode 100644
index c532e9f5d1..0000000000
--- a/code/modules/tgui/states/default_contained.dm
+++ /dev/null
@@ -1,13 +0,0 @@
-/**
- * tgui state: default_contained
- *
- * Basically default and contained combined, allowing for both
- */
-
-GLOBAL_DATUM_INIT(default_contained_state, /datum/ui_state/default/contained, new)
-
-/datum/ui_state/default/contained/can_use_topic(atom/src_object, mob/user)
-	if(src_object.contains(user))
-		return UI_INTERACTIVE
-	return ..()
-	
\ No newline at end of file
diff --git a/code/modules/tgui/states/hands.dm b/code/modules/tgui/states/hands.dm
index d73d1058ea..1c885ed414 100644
--- a/code/modules/tgui/states/hands.dm
+++ b/code/modules/tgui/states/hands.dm
@@ -2,6 +2,9 @@
  * tgui state: hands_state
  *
  * Checks that the src_object is in the user's hands.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(hands_state, /datum/ui_state/hands_state, new)
@@ -19,7 +22,7 @@ GLOBAL_DATUM_INIT(hands_state, /datum/ui_state/hands_state, new)
 		return UI_INTERACTIVE
 	return UI_CLOSE
 
-/mob/living/silicon/robot/hands_can_use_topic(obj/src_object)
-	if(activated(src_object) || istype(src_object.loc, /obj/item/weapon/gripper))
+/mob/living/silicon/robot/hands_can_use_topic(src_object)
+	if(activated(src_object))
 		return UI_INTERACTIVE
 	return UI_CLOSE
diff --git a/code/modules/tgui/states/human_adjacent.dm b/code/modules/tgui/states/human_adjacent.dm
index 7aefe43e44..2ac7c8637b 100644
--- a/code/modules/tgui/states/human_adjacent.dm
+++ b/code/modules/tgui/states/human_adjacent.dm
@@ -3,6 +3,9 @@
  *
  * In addition to default checks, only allows interaction for a
  * human adjacent user.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(human_adjacent_state, /datum/ui_state/human_adjacent_state, new)
diff --git a/code/modules/tgui/states/inventory.dm b/code/modules/tgui/states/inventory.dm
index 43fe2cb451..dc5dd0d57e 100644
--- a/code/modules/tgui/states/inventory.dm
+++ b/code/modules/tgui/states/inventory.dm
@@ -1,7 +1,11 @@
 /**
  * tgui state: inventory_state
  *
- * Checks that the src_object is in the user's top-level (hand, ear, pocket, belt, etc) inventory.
+ * Checks that the src_object is in the user's top-level
+ * (hand, ear, pocket, belt, etc) inventory.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(inventory_state, /datum/ui_state/inventory_state, new)
diff --git a/code/modules/tgui/states/language_menu.dm b/code/modules/tgui/states/language_menu.dm
index 5c816c8922..6389b05cd5 100644
--- a/code/modules/tgui/states/language_menu.dm
+++ b/code/modules/tgui/states/language_menu.dm
@@ -1,5 +1,8 @@
 /**
  * tgui state: language_menu_state
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(language_menu_state, /datum/ui_state/language_menu, new)
diff --git a/code/modules/tgui/states/not_incapacitated.dm b/code/modules/tgui/states/not_incapacitated.dm
index 364b59424d..16dcb7881e 100644
--- a/code/modules/tgui/states/not_incapacitated.dm
+++ b/code/modules/tgui/states/not_incapacitated.dm
@@ -2,6 +2,9 @@
  * tgui state: not_incapacitated_state
  *
  * Checks that the user isn't incapacitated
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(not_incapacitated_state, /datum/ui_state/not_incapacitated_state, new)
@@ -24,6 +27,10 @@ GLOBAL_DATUM_INIT(not_incapacitated_turf_state, /datum/ui_state/not_incapacitate
 /datum/ui_state/not_incapacitated_state/can_use_topic(src_object, mob/user)
 	if(user.stat)
 		return UI_CLOSE
-	if(user.incapacitated() || user.lying || (turf_check && !isturf(user.loc)))
+	if(user.incapacitated() || (turf_check && !isturf(user.loc)))
 		return UI_DISABLED
-	return UI_INTERACTIVE
\ No newline at end of file
+	if(isliving(user))
+		var/mob/living/L = user
+		if(!(L.mobility_flags & MOBILITY_STAND))
+			return UI_DISABLED
+	return UI_INTERACTIVE
diff --git a/code/modules/tgui/states/notcontained.dm b/code/modules/tgui/states/notcontained.dm
index 642c6ce95f..1d4e6aec19 100644
--- a/code/modules/tgui/states/notcontained.dm
+++ b/code/modules/tgui/states/notcontained.dm
@@ -1,7 +1,11 @@
 /**
  * tgui state: notcontained_state
  *
- * Checks that the user is not inside src_object, and then makes the default checks.
+ * Checks that the user is not inside src_object, and then makes the
+ * default checks.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(notcontained_state, /datum/ui_state/notcontained_state, new)
diff --git a/code/modules/tgui/states/observer.dm b/code/modules/tgui/states/observer.dm
index 86ad776b13..d105de1c0c 100644
--- a/code/modules/tgui/states/observer.dm
+++ b/code/modules/tgui/states/observer.dm
@@ -2,6 +2,9 @@
  * tgui state: observer_state
  *
  * Checks that the user is an observer/ghost.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(observer_state, /datum/ui_state/observer_state, new)
diff --git a/code/modules/tgui/states/physical.dm b/code/modules/tgui/states/physical.dm
index 88c8a291aa..3073039d14 100644
--- a/code/modules/tgui/states/physical.dm
+++ b/code/modules/tgui/states/physical.dm
@@ -2,6 +2,9 @@
  * tgui state: physical_state
  *
  * Short-circuits the default state to only check physical distance.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(physical_state, /datum/ui_state/physical, new)
diff --git a/code/modules/tgui/states/self.dm b/code/modules/tgui/states/self.dm
index b0c9500fbc..4b6e3b9fd9 100644
--- a/code/modules/tgui/states/self.dm
+++ b/code/modules/tgui/states/self.dm
@@ -2,6 +2,9 @@
  * tgui state: self_state
  *
  * Only checks that the user and src_object are the same.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(self_state, /datum/ui_state/self_state, new)
diff --git a/code/modules/tgui/states/zlevel.dm b/code/modules/tgui/states/zlevel.dm
index 5e3ccfb7de..64ea2fa1c0 100644
--- a/code/modules/tgui/states/zlevel.dm
+++ b/code/modules/tgui/states/zlevel.dm
@@ -2,6 +2,9 @@
  * tgui state: z_state
  *
  * Only checks that the Z-level of the user and src_object are the same.
+ *
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 GLOBAL_DATUM_INIT(z_state, /datum/ui_state/z_state, new)
diff --git a/code/modules/tgui/subsystem.dm b/code/modules/tgui/subsystem.dm
deleted file mode 100644
index cbe94e2d7f..0000000000
--- a/code/modules/tgui/subsystem.dm
+++ /dev/null
@@ -1,247 +0,0 @@
-/**
- * tgui subsystem
- *
- * Contains all tgui state and subsystem code.
- */
-
-/**
- * public
- *
- * Get a open UI given a user, src_object, and ui_key and try to update it with data.
- *
- * required user mob The mob who opened/is using the UI.
- * required src_object datum The object/datum which owns the UI.
- * required ui_key string The ui_key of the UI.
- * optional ui datum/tgui The UI to be updated, if it exists.
- * optional force_open bool If the UI should be re-opened instead of updated.
- *
- * return datum/tgui The found UI.
- */
-/datum/controller/subsystem/tgui/proc/try_update_ui(mob/user, datum/src_object, ui_key, datum/tgui/ui, force_open = FALSE)
-	if(isnull(ui)) // No UI was passed, so look for one.
-		ui = get_open_ui(user, src_object, ui_key)
-
-	if(!isnull(ui))
-		var/data = src_object.ui_data(user) // Get data from the src_object.
-		if(!force_open) // UI is already open; update it.
-			ui.push_data(data)
-		else // Re-open it anyways.
-			ui.reinitialize(null, data)
-		return ui // We found the UI, return it.
-	else
-		return null // We couldn't find a UI.
-
-/**
- * private
- *
- * Get a open UI given a user, src_object, and ui_key.
- *
- * required user mob The mob who opened/is using the UI.
- * required src_object datum The object/datum which owns the UI.
- * required ui_key string The ui_key of the UI.
- *
- * return datum/tgui The found UI.
- */
-/datum/controller/subsystem/tgui/proc/get_open_ui(mob/user, datum/src_object, ui_key)
-	var/src_object_key = "[REF(src_object)]"
-	if(isnull(open_uis[src_object_key]) || !istype(open_uis[src_object_key], /list))
-		return null // No UIs open.
-	else if(isnull(open_uis[src_object_key][ui_key]) || !istype(open_uis[src_object_key][ui_key], /list))
-		return null // No UIs open for this object.
-
-	for(var/datum/tgui/ui in open_uis[src_object_key][ui_key]) // Find UIs for this object.
-		if(ui.user == user) // Make sure we have the right user
-			return ui
-
-	return null // Couldn't find a UI!
-
-/**
- * private
- *
- * Update all UIs attached to src_object.
- *
- * required src_object datum The object/datum which owns the UIs.
- *
- * return int The number of UIs updated.
- */
-/datum/controller/subsystem/tgui/proc/update_uis(datum/src_object)
-	var/src_object_key = "[REF(src_object)]"
-	if(isnull(open_uis[src_object_key]) || !istype(open_uis[src_object_key], /list))
-		return 0 // Couldn't find any UIs for this object.
-
-	var/update_count = 0
-	for(var/ui_key in open_uis[src_object_key])
-		for(var/datum/tgui/ui in open_uis[src_object_key][ui_key])
-			if(ui && ui.src_object && ui.user && ui.src_object.ui_host(ui.user)) // Check the UI is valid.
-				ui.process(force = 1) // Update the UI.
-				update_count++ // Count each UI we update.
-	return update_count
-
-/**
- * private
- *
- * Close all UIs attached to src_object.
- *
- * required src_object datum The object/datum which owns the UIs.
- *
- * return int The number of UIs closed.
- */
-/datum/controller/subsystem/tgui/proc/close_uis(datum/src_object)
-	var/src_object_key = "[REF(src_object)]"
-	if(isnull(open_uis[src_object_key]) || !istype(open_uis[src_object_key], /list))
-		return 0 // Couldn't find any UIs for this object.
-
-	var/close_count = 0
-	for(var/ui_key in open_uis[src_object_key])
-		for(var/datum/tgui/ui in open_uis[src_object_key][ui_key])
-			if(ui && ui.src_object && ui.user && ui.src_object.ui_host(ui.user)) // Check the UI is valid.
-				ui.close() // Close the UI.
-				close_count++ // Count each UI we close.
-	return close_count
-
-/**
- * private
- *
- * Close *ALL* UIs
- *
- * return int The number of UIs closed.
- */
-/datum/controller/subsystem/tgui/proc/close_all_uis()
-	var/close_count = 0
-	for(var/src_object_key in open_uis)
-		for(var/ui_key in open_uis[src_object_key])
-			for(var/datum/tgui/ui in open_uis[src_object_key][ui_key])
-				if(ui && ui.src_object && ui.user && ui.src_object.ui_host(ui.user)) // Check the UI is valid.
-					ui.close() // Close the UI.
-					close_count++ // Count each UI we close.
-	return close_count
-
-/**
- * private
- *
- * Update all UIs belonging to a user.
- *
- * required user mob The mob who opened/is using the UI.
- * optional src_object datum If provided, only update UIs belonging this src_object.
- * optional ui_key string If provided, only update UIs with this UI key.
- *
- * return int The number of UIs updated.
- */
-/datum/controller/subsystem/tgui/proc/update_user_uis(mob/user, datum/src_object = null, ui_key = null)
-	if(isnull(user.open_uis) || !istype(user.open_uis, /list) || open_uis.len == 0)
-		return 0 // Couldn't find any UIs for this user.
-
-	var/update_count = 0
-	for(var/datum/tgui/ui in user.open_uis)
-		if((isnull(src_object) || !isnull(src_object) && ui.src_object == src_object) && (isnull(ui_key) || !isnull(ui_key) && ui.ui_key == ui_key))
-			ui.process(force = 1) // Update the UI.
-			update_count++ // Count each UI we upadte.
-	return update_count
-
-/**
- * private
- *
- * Close all UIs belonging to a user.
- *
- * required user mob The mob who opened/is using the UI.
- * optional src_object datum If provided, only close UIs belonging this src_object.
- * optional ui_key string If provided, only close UIs with this UI key.
- *
- * return int The number of UIs closed.
- */
-/datum/controller/subsystem/tgui/proc/close_user_uis(mob/user, datum/src_object = null, ui_key = null)
-	if(isnull(user.open_uis) || !istype(user.open_uis, /list) || open_uis.len == 0)
-		return 0 // Couldn't find any UIs for this user.
-
-	var/close_count = 0
-	for(var/datum/tgui/ui in user.open_uis)
-		if((isnull(src_object) || !isnull(src_object) && ui.src_object == src_object) && (isnull(ui_key) || !isnull(ui_key) && ui.ui_key == ui_key))
-			ui.close() // Close the UI.
-			close_count++ // Count each UI we close.
-	return close_count
-
-/**
- * private
- *
- * Add a UI to the list of open UIs.
- *
- * required ui datum/tgui The UI to be added.
- */
-/datum/controller/subsystem/tgui/proc/on_open(datum/tgui/ui)
-	var/src_object_key = "[REF(ui.src_object)]"
-	if(isnull(open_uis[src_object_key]) || !istype(open_uis[src_object_key], /list))
-		open_uis[src_object_key] = list(ui.ui_key = list()) // Make a list for the ui_key and src_object.
-	else if(isnull(open_uis[src_object_key][ui.ui_key]) || !istype(open_uis[src_object_key][ui.ui_key], /list))
-		open_uis[src_object_key][ui.ui_key] = list() // Make a list for the ui_key.
-
-	// Append the UI to all the lists.
-	ui.user.open_uis |= ui
-	var/list/uis = open_uis[src_object_key][ui.ui_key]
-	uis |= ui
-	processing_uis |= ui
-
-/**
- * private
- *
- * Remove a UI from the list of open UIs.
- *
- * required ui datum/tgui The UI to be removed.
- *
- * return bool If the UI was removed or not.
- */
-/datum/controller/subsystem/tgui/proc/on_close(datum/tgui/ui)
-	var/src_object_key = "[REF(ui.src_object)]"
-	if(isnull(open_uis[src_object_key]) || !istype(open_uis[src_object_key], /list))
-		return 0 // It wasn't open.
-	else if(isnull(open_uis[src_object_key][ui.ui_key]) || !istype(open_uis[src_object_key][ui.ui_key], /list))
-		return 0 // It wasn't open.
-
-	processing_uis.Remove(ui) // Remove it from the list of processing UIs.
-	if(ui.user)	// If the user exists, remove it from them too.
-		ui.user.open_uis.Remove(ui)
-	var/Ukey = ui.ui_key
-	var/list/uis = open_uis[src_object_key][Ukey] // Remove it from the list of open UIs.
-	uis.Remove(ui)
-	if(!uis.len)
-		var/list/uiobj = open_uis[src_object_key]
-		uiobj.Remove(Ukey)
-		if(!uiobj.len)
-			open_uis.Remove(src_object_key)
-
-	return 1 // Let the caller know we did it.
-
-/**
- * private
- *
- * Handle client logout, by closing all their UIs.
- *
- * required user mob The mob which logged out.
- *
- * return int The number of UIs closed.
- */
-/datum/controller/subsystem/tgui/proc/on_logout(mob/user)
-	return close_user_uis(user)
-
-/**
- * private
- *
- * Handle clients switching mobs, by transferring their UIs.
- *
- * required user source The client's original mob.
- * required user target The client's new mob.
- *
- * return bool If the UIs were transferred.
- */
-/datum/controller/subsystem/tgui/proc/on_transfer(mob/source, mob/target)
-	if(!source || isnull(source.open_uis) || !istype(source.open_uis, /list) || open_uis.len == 0)
-		return 0 // The old mob had no open UIs.
-
-	if(isnull(target.open_uis) || !istype(target.open_uis, /list))
-		target.open_uis = list() // Create a list for the new mob if needed.
-
-	for(var/datum/tgui/ui in source.open_uis)
-		ui.user = target // Inform the UIs of their new owner.
-		target.open_uis.Add(ui) // Transfer all the UIs.
-
-	source.open_uis.Cut() // Clear the old list.
-	return 1 // Let the caller know we did it.
diff --git a/code/modules/tgui/tgui.dm b/code/modules/tgui/tgui.dm
index 7971a940d4..5eacfd07fe 100644
--- a/code/modules/tgui/tgui.dm
+++ b/code/modules/tgui/tgui.dm
@@ -1,7 +1,6 @@
 /**
- * tgui
- *
- * /tg/station user interface library
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
  */
 
 /**
@@ -14,34 +13,26 @@
 	var/datum/src_object
 	/// The title of te UI.
 	var/title
-	/// The ui_key of the UI. This allows multiple UIs for one src_object.
-	var/ui_key
 	/// The window_id for browse() and onclose().
-	var/window_id
-	/// The window width.
-	var/width = 0
-	/// The window height
-	var/height = 0
+	var/datum/tgui_window/window
+	/// Key that is used for remembering the window geometry.
+	var/window_key
+	/// Deprecated: Window size.
+	var/window_size
 	/// The interface (template) to be used for this UI.
 	var/interface
 	/// Update the UI every MC tick.
 	var/autoupdate = TRUE
 	/// If the UI has been initialized yet.
 	var/initialized = FALSE
-	/// The data (and datastructure) used to initialize the UI.
-	var/list/initial_data
-	/// The static data used to initialize the UI.
-	var/list/initial_static_data
-	/// Holder for the json string, that is sent during the initial update
-	var/_initial_update
+	/// Time of opening the window.
+	var/opened_at
+	/// Stops further updates when close() was called.
+	var/closing = FALSE
 	/// The status/visibility of the UI.
 	var/status = UI_INTERACTIVE
 	/// Topic state used to determine status/interactability.
 	var/datum/ui_state/state = null
-	/// The parent UI.
-	var/datum/tgui/master_ui
-	/// Children of this UI.
-	var/list/datum/tgui/children = list()
 
 /**
  * public
@@ -50,38 +41,25 @@
  *
  * required user mob The mob who opened/is using the UI.
  * required src_object datum The object or datum which owns the UI.
- * required ui_key string The ui_key of the UI.
  * required interface string The interface used to render the UI.
  * optional title string The title of the UI.
- * optional width int The window width.
- * optional height int The window height.
- * optional master_ui datum/tgui The parent UI.
- * optional state datum/ui_state The state used to determine status.
+ * optional ui_x int Deprecated: Window width.
+ * optional ui_y int Deprecated: Window height.
  *
  * return datum/tgui The requested UI.
  */
-/datum/tgui/New(mob/user, datum/src_object, ui_key, interface, title, width = 0, height = 0, datum/tgui/master_ui = null, datum/ui_state/state = GLOB.default_state)
+/datum/tgui/New(mob/user, datum/src_object, interface, title, ui_x, ui_y)
+	log_tgui(user, "new [interface] fancy [user.client.prefs.tgui_fancy]")
 	src.user = user
 	src.src_object = src_object
-	src.ui_key = ui_key
-	// DO NOT replace with \ref here. src_object could potentially be tagged
-	src.window_id = "[REF(src_object)]-[ui_key]"
+	src.window_key = "[REF(src_object)]-main"
 	src.interface = interface
-
 	if(title)
-		src.title = sanitize(title)
-	if(width)
-		src.width = width
-	if(height)
-		src.height = height
-
-	src.master_ui = master_ui
-	if(master_ui)
-		master_ui.children += src
-	src.state = state
-
-	var/datum/asset/assets = get_asset_datum(/datum/asset/group/tgui)
-	assets.send(user)
+		src.title = title
+	src.state = src_object.ui_state()
+	// Deprecated
+	if(ui_x && ui_y)
+		src.window_size = list(ui_x, ui_y)
 
 /**
  * public
@@ -90,85 +68,53 @@
  */
 /datum/tgui/proc/open()
 	if(!user.client)
-		return // Bail if there is no client.
-
-	update_status(push = FALSE) // Update the window status.
+		return null
+	if(window)
+		return null
+	process_status()
 	if(status < UI_UPDATE)
-		return // Bail if we're not supposed to open.
-
-	// Build window options
-	var/window_options = "can_minimize=0;auto_format=0;"
-	// If we have a width and height, use them.
-	if(width && height)
-		window_options += "size=[width]x[height];"
-	// Remove titlebar and resize handles for a fancy window
-	if(user.client.prefs.tgui_fancy)
-		window_options += "titlebar=0;can_resize=0;"
+		return null
+	window = SStgui.request_pooled_window(user)
+	if(!window)
+		return null
+	opened_at = world.time
+	window.acquire_lock(src)
+	if(!window.is_ready())
+		window.initialize(inline_assets = list(
+			get_asset_datum(/datum/asset/simple/tgui),
+		))
 	else
-		window_options += "titlebar=1;can_resize=1;"
-
-	// Generate page html
-	var/html
-	html = SStgui.basehtml
-	// Allow the src object to override the html if needed
-	html = src_object.ui_base_html(html)
-	// Replace template tokens with important UI data
-	// NOTE: Intentional \ref usage; tgui datums can't/shouldn't
-	// be tagged, so this is an effective unwrap
-	html = replacetextEx(html, "\[tgui:ref]", "\ref[src]")
-
-	// Open the window.
-	user << browse(html, "window=[window_id];[window_options]")
-
-	// Instruct the client to signal UI when the window is closed.
-	// NOTE: Intentional \ref usage; tgui datums can't/shouldn't
-	// be tagged, so this is an effective unwrap
-	winset(user, window_id, "on-close=\"uiclose \ref[src]\"")
-
-	// Pre-fetch initial state while browser is still loading in
-	// another thread
-	if(!initial_data) {
-		initial_data = src_object.ui_data(user)
-	}
-	if(!initial_static_data) {
-		initial_static_data = src_object.ui_static_data(user)
-	}
-	_initial_update = url_encode(get_json(initial_data, initial_static_data))
-
+		window.send_message("ping")
+	window.send_asset(get_asset_datum(/datum/asset/simple/fontawesome))
+	for(var/datum/asset/asset in src_object.ui_assets(user))
+		window.send_asset(asset)
+	window.send_message("update", get_payload(
+		with_data = TRUE,
+		with_static_data = TRUE))
 	SStgui.on_open(src)
 
 /**
  * public
  *
- * Reinitialize the UI.
- * (Possibly with a new interface and/or data).
+ * Close the UI.
  *
- * optional template string The name of the new interface.
- * optional data list The new initial data.
+ * optional can_be_suspended bool
  */
-/datum/tgui/proc/reinitialize(interface, list/data, list/static_data)
-	if(interface)
-		src.interface = interface
-	if(data)
-		initial_data = data
-	if(static_data)
-		initial_static_data = static_data
-	open()
-
-/**
- * public
- *
- * Close the UI, and all its children.
- */
-/datum/tgui/proc/close()
-	user << browse(null, "window=[window_id]") // Close the window.
-	src_object.ui_close(user)
-	SStgui.on_close(src)
-	for(var/datum/tgui/child in children) // Loop through and close all children.
-		child.close()
-	children.Cut()
+/datum/tgui/proc/close(can_be_suspended = TRUE)
+	if(closing)
+		return
+	closing = TRUE
+	// If we don't have window_id, open proc did not have the opportunity
+	// to finish, therefore it's safe to skip this whole block.
+	if(window)
+		// Windows you want to keep are usually blue screens of death
+		// and we want to keep them around, to allow user to read
+		// the error message properly.
+		window.release_lock()
+		window.close(can_be_suspended)
+		src_object.ui_close(user)
+		SStgui.on_close(src)
 	state = null
-	master_ui = null
 	qdel(src)
 
 /**
@@ -176,187 +122,173 @@
  *
  * Enable/disable auto-updating of the UI.
  *
- * required state bool Enable/disable auto-updating.
+ * required value bool Enable/disable auto-updating.
  */
-/datum/tgui/proc/set_autoupdate(state = TRUE)
-	autoupdate = state
+/datum/tgui/proc/set_autoupdate(autoupdate)
+	src.autoupdate = autoupdate
+
+/**
+ * public
+ *
+ * Replace current ui.state with a new one.
+ *
+ * required state datum/ui_state/state Next state
+ */
+/datum/tgui/proc/set_state(datum/ui_state/state)
+	src.state = state
+
+/**
+ * public
+ *
+ * Makes an asset available to use in tgui.
+ *
+ * required asset datum/asset
+ */
+/datum/tgui/proc/send_asset(datum/asset/asset)
+	if(!window)
+		CRASH("send_asset() can only be called after open().")
+	window.send_asset(asset)
+
+/**
+ * public
+ *
+ * Send a full update to the client (includes static data).
+ *
+ * optional custom_data list Custom data to send instead of ui_data.
+ * optional force bool Send an update even if UI is not interactive.
+ */
+/datum/tgui/proc/send_full_update(custom_data, force)
+	if(!user.client || !initialized || closing)
+		return
+	var/should_update_data = force || status >= UI_UPDATE
+	window.send_message("update", get_payload(
+		custom_data,
+		with_data = should_update_data,
+		with_static_data = TRUE))
+
+/**
+ * public
+ *
+ * Send a partial update to the client (excludes static data).
+ *
+ * optional custom_data list Custom data to send instead of ui_data.
+ * optional force bool Send an update even if UI is not interactive.
+ */
+/datum/tgui/proc/send_update(custom_data, force)
+	if(!user.client || !initialized || closing)
+		return
+	var/should_update_data = force || status >= UI_UPDATE
+	window.send_message("update", get_payload(
+		custom_data,
+		with_data = should_update_data))
 
 /**
  * private
  *
  * Package the data to send to the UI, as JSON.
- * This includes the UI data and config_data.
  *
- * return string The packaged JSON.
+ * return list
  */
-/datum/tgui/proc/get_json(list/data, list/static_data)
+/datum/tgui/proc/get_payload(custom_data, with_data, with_static_data)
 	var/list/json_data = list()
-
 	json_data["config"] = list(
 		"title" = title,
 		"status" = status,
 		"interface" = interface,
-		"fancy" = user.client.prefs.tgui_fancy,
-		"locked" = user.client.prefs.tgui_lock,
-		"observer" = isobserver(user),
-		"window" = window_id,
-		// NOTE: Intentional \ref usage; tgui datums can't/shouldn't
-		// be tagged, so this is an effective unwrap
-		"ref" = "\ref[src]"
+		"window" = list(
+			"key" = window_key,
+			"size" = window_size,
+			"fancy" = user.client.prefs.tgui_fancy,
+			"locked" = user.client.prefs.tgui_lock,
+		),
+		"user" = list(
+			"name" = "[user]",
+			"ckey" = "[user.ckey]",
+			"observer" = isobserver(user),
+		),
 	)
-
-	if(!isnull(data))
+	var/data = custom_data || with_data && src_object.ui_data(user)
+	if(data)
 		json_data["data"] = data
-	if(!isnull(static_data))
+	var/static_data = with_static_data && src_object.ui_static_data(user)
+	if(static_data)
 		json_data["static_data"] = static_data
-
-	// Send shared states
 	if(src_object.tgui_shared_states)
 		json_data["shared"] = src_object.tgui_shared_states
-
-	// Generate the JSON.
-	var/json = json_encode(json_data)
-	// Strip #255/improper.
-	json = replacetext(json, "\proper", "")
-	json = replacetext(json, "\improper", "")
-	return json
+	return json_data
 
 /**
  * private
  *
- * Handle clicks from the UI.
- * Call the src_object's ui_act() if status is UI_INTERACTIVE.
- * If the src_object's ui_act() returns 1, update all UIs attacked to it.
- */
-/datum/tgui/Topic(href, href_list)
-	if(user != usr)
-		return // Something is not right here.
-
-	var/action = href_list["action"]
-	var/params = href_list; params -= "action"
-
-	switch(action)
-		if("tgui:initialize")
-			user << output(_initial_update, "[window_id].browser:update")
-			initialized = TRUE
-		if("tgui:setSharedState")
-			// Update the window state.
-			update_status(push = FALSE)
-			// Bail if UI is not interactive or usr calling Topic
-			// is not the UI user.
-			if(status != UI_INTERACTIVE)
-				return
-			var/key = params["key"]
-			var/value = params["value"]
-			if(!src_object.tgui_shared_states)
-				src_object.tgui_shared_states = list()
-			src_object.tgui_shared_states[key] = value
-			SStgui.update_uis(src_object)
-		if("tgui:setFancy")
-			var/value = text2num(params["value"])
-			user.client.prefs.tgui_fancy = value
-		if("tgui:log")
-			// Force window to show frills on fatal errors
-			if(params["fatal"])
-				winset(user, window_id, "titlebar=1;can-resize=1;size=600x600")
-			log_message(params["log"])
-		if("tgui:link")
-			user << link(params["url"])
-		else
-			// Update the window state.
-			update_status(push = FALSE)
-			// Call ui_act() on the src_object.
-			if(src_object.ui_act(action, params, src, state))
-				// Update if the object requested it.
-				SStgui.update_uis(src_object)
-
-/**
- * private
- *
- * Update the UI.
- * Only updates the data if update is true, otherwise only updates the status.
- *
- * optional force bool If the UI should be forced to update.
+ * Run an update cycle for this UI. Called internally by SStgui
+ * every second or so.
  */
 /datum/tgui/process(force = FALSE)
+	if(closing)
+		return
 	var/datum/host = src_object.ui_host(user)
-	if(!src_object || !host || !user) // If the object or user died (or something else), abort.
+	// If the object or user died (or something else), abort.
+	if(!src_object || !host || !user || !window)
+		close(can_be_suspended = FALSE)
+		return
+	// Validate ping
+	if(!initialized && world.time - opened_at > TGUI_PING_TIMEOUT)
+		log_tgui(user, \
+			"Error: Zombie window detected, killing it with fire.\n" \
+			+ "window_id: [window.id]\n" \
+			+ "opened_at: [opened_at]\n" \
+			+ "world.time: [world.time]")
+		close(can_be_suspended = FALSE)
+		return
+	// Update through a normal call to ui_interact
+	if(status != UI_DISABLED && (autoupdate || force))
+		src_object.ui_interact(user, src)
+		return
+	// Update status only
+	var/needs_update = process_status()
+	if(status <= UI_CLOSE)
 		close()
 		return
-
-	if(status && (force || autoupdate))
-		update() // Update the UI if the status and update settings allow it.
-	else
-		update_status(push = TRUE) // Otherwise only update status.
+	if(needs_update)
+		window.send_message("update", get_payload())
 
 /**
  * private
  *
- * Push data to an already open UI.
- *
- * required data list The data to send.
- * optional force bool If the update should be sent regardless of state.
+ * Updates the status, and returns TRUE if status has changed.
  */
-/datum/tgui/proc/push_data(data, static_data, force = FALSE)
-	// Update the window state.
-	update_status(push = FALSE)
-	// Cannot update UI if it is not set up yet.
-	if(!initialized)
-		return
-	// Cannot update UI, we have no visibility.
-	if(status <= UI_DISABLED && !force)
-		return
-	// Send the new JSON to the update() Javascript function.
-	user << output(
-		url_encode(get_json(data, static_data)),
-		"[window_id].browser:update")
+/datum/tgui/proc/process_status()
+	var/prev_status = status
+	status = src_object.ui_status(user, state)
+	return prev_status != status
 
 /**
  * private
  *
- * Updates the UI by interacting with the src_object again, which will hopefully
- * call try_ui_update on it.
- *
- * optional force_open bool If force_open should be passed to ui_interact.
+ * Callback for handling incoming tgui messages.
  */
-/datum/tgui/proc/update(force_open = FALSE)
-	src_object.ui_interact(user, ui_key, src, force_open, master_ui, state)
-
-/**
- * private
- *
- * Update the status/visibility of the UI for its user.
- *
- * optional push bool Push an update to the UI (an update is always sent for UI_DISABLED).
- */
-/datum/tgui/proc/update_status(push = FALSE)
-	var/status = src_object.ui_status(user, state)
-	if(master_ui)
-		status = min(status, master_ui.status)
-	set_status(status, push)
-	if(status == UI_CLOSE)
-		close()
-
-/**
- * private
- *
- * Set the status/visibility of the UI.
- *
- * required status int The status to set (UI_CLOSE/UI_DISABLED/UI_UPDATE/UI_INTERACTIVE).
- * optional push bool Push an update to the UI (an update is always sent for UI_DISABLED).
- */
-/datum/tgui/proc/set_status(status, push = FALSE)
-	// Only update if status has changed.
-	if(src.status != status)
-		if(src.status == UI_DISABLED)
-			src.status = status
-			if(push)
-				update()
-		else
-			src.status = status
-			// Update if the UI just because disabled, or a push is requested.
-			if(status == UI_DISABLED || push)
-				push_data(null, force = TRUE)
-
-/datum/tgui/proc/log_message(message)
-	log_tgui("[user] ([user.ckey]) using \"[title]\":\n[message]")
+/datum/tgui/proc/on_message(type, list/payload, list/href_list)
+	// Pass act type messages to ui_act
+	if(type && copytext(type, 1, 5) == "act/")
+		process_status()
+		if(src_object.ui_act(copytext(type, 5), payload, src, state))
+			SStgui.update_uis(src_object)
+		return FALSE
+	switch(type)
+		if("ready")
+			initialized = TRUE
+		if("pingReply")
+			initialized = TRUE
+		if("suspend")
+			close(can_be_suspended = TRUE)
+		if("close")
+			close(can_be_suspended = FALSE)
+		if("log")
+			if(href_list["fatal"])
+				close(can_be_suspended = FALSE)
+		if("setSharedState")
+			if(status != UI_INTERACTIVE)
+				return
+			LAZYINITLIST(src_object.tgui_shared_states)
+			src_object.tgui_shared_states[href_list["key"]] = href_list["value"]
+			SStgui.update_uis(src_object)
diff --git a/code/modules/tgui/tgui_window.dm b/code/modules/tgui/tgui_window.dm
new file mode 100644
index 0000000000..3f271163c9
--- /dev/null
+++ b/code/modules/tgui/tgui_window.dm
@@ -0,0 +1,238 @@
+/**
+ * Copyright (c) 2020 Aleksej Komarov
+ * SPDX-License-Identifier: MIT
+ */
+
+/datum/tgui_window
+	var/id
+	var/client/client
+	var/pooled
+	var/pool_index
+	var/status = TGUI_WINDOW_CLOSED
+	var/locked = FALSE
+	var/datum/tgui/locked_by
+	var/fatally_errored = FALSE
+	var/message_queue
+	var/sent_assets = list()
+
+/**
+ * public
+ *
+ * Create a new tgui window.
+ *
+ * required client /client
+ * required id string A unique window identifier.
+ */
+/datum/tgui_window/New(client/client, id, pooled = FALSE)
+	src.id = id
+	src.client = client
+	src.pooled = pooled
+	if(pooled)
+		client.tgui_windows[id] = src
+		src.pool_index = TGUI_WINDOW_INDEX(id)
+
+/**
+ * public
+ *
+ * Initializes the window with a fresh page. Puts window into the "loading"
+ * state. You can begin sending messages right after initializing. Messages
+ * will be put into the queue until the window finishes loading.
+ *
+ * optional inline_assets list List of assets to inline into the html.
+ */
+/datum/tgui_window/proc/initialize(inline_assets = list())
+	log_tgui(client, "[id]/initialize")
+	if(!client)
+		return
+	status = TGUI_WINDOW_LOADING
+	fatally_errored = FALSE
+	message_queue = null
+	// Build window options
+	var/options = "file=[id].html;can_minimize=0;auto_format=0;"
+	// Remove titlebar and resize handles for a fancy window
+	if(client.prefs.tgui_fancy)
+		options += "titlebar=0;can_resize=0;"
+	else
+		options += "titlebar=1;can_resize=1;"
+	// Generate page html
+	var/html = SStgui.basehtml
+	html = replacetextEx(html, "\[tgui:windowId]", id)
+	// Process inline assets
+	var/inline_styles = ""
+	var/inline_scripts = ""
+	for(var/datum/asset/asset in inline_assets)
+		var/mappings = asset.get_url_mappings()
+		for(var/name in mappings)
+			var/url = mappings[name]
+			// Not urlencoding since asset strings are considered safe
+			if(copytext(name, -4) == ".css")
+				inline_styles += "<link rel=\"stylesheet\" type=\"text/css\" href=\"[url]\">\n"
+			else if(copytext(name, -3) == ".js")
+				inline_scripts += "<script type=\"text/javascript\" defer src=\"[url]\"></script>\n"
+		asset.send()
+	html = replacetextEx(html, "<!-- tgui:styles -->\n", inline_styles)
+	html = replacetextEx(html, "<!-- tgui:scripts -->\n", inline_scripts)
+	// Open the window
+	client << browse(html, "window=[id];[options]")
+	// Instruct the client to signal UI when the window is closed.
+	winset(client, id, "on-close=\"uiclose [id]\"")
+
+/**
+ * public
+ *
+ * Checks if the window is ready to receive data.
+ *
+ * return bool
+ */
+/datum/tgui_window/proc/is_ready()
+	return status == TGUI_WINDOW_READY
+
+/**
+ * public
+ *
+ * Checks if the window can be sanely suspended.
+ *
+ * return bool
+ */
+/datum/tgui_window/proc/can_be_suspended()
+	return !fatally_errored \
+		&& pooled \
+		&& pool_index > 0 \
+		&& pool_index <= TGUI_WINDOW_SOFT_LIMIT \
+		&& status == TGUI_WINDOW_READY
+
+/**
+ * public
+ *
+ * Acquire the window lock. Pool will not be able to provide this window
+ * to other UIs for the duration of the lock.
+ *
+ * Can be given an optional tgui datum, which will hook its on_message
+ * callback into the message stream.
+ *
+ * optional ui /datum/tgui
+ */
+/datum/tgui_window/proc/acquire_lock(datum/tgui/ui)
+	locked = TRUE
+	locked_by = ui
+
+/**
+ * Release the window lock.
+ */
+/datum/tgui_window/proc/release_lock()
+	// Clean up assets sent by tgui datum which requested the lock
+	if(locked)
+		sent_assets = list()
+	locked = FALSE
+	locked_by = null
+
+/**
+ * public
+ *
+ * Close the UI.
+ *
+ * optional can_be_suspended bool
+ */
+/datum/tgui_window/proc/close(can_be_suspended = TRUE)
+	if(!client)
+		return
+	if(can_be_suspended && can_be_suspended())
+		log_tgui(client, "[id]/close: suspending")
+		status = TGUI_WINDOW_READY
+		send_message("suspend")
+		return
+	log_tgui(client, "[id]/close")
+	release_lock()
+	status = TGUI_WINDOW_CLOSED
+	message_queue = null
+	// Do not close the window to give user some time
+	// to read the error message.
+	if(!fatally_errored)
+		client << browse(null, "window=[id]")
+
+/**
+ * public
+ *
+ * Sends a message to tgui window.
+ *
+ * required type string Message type
+ * required payload list Message payload
+ * optional force bool Send regardless of the ready status.
+ */
+/datum/tgui_window/proc/send_message(type, list/payload, force)
+	if(!client)
+		return
+	var/message = json_encode(list(
+		"type" = type,
+		"payload" = payload,
+	))
+	// Strip #255/improper.
+	message = replacetext(message, "\proper", "")
+	message = replacetext(message, "\improper", "")
+	// Pack for sending via output()
+	message = url_encode(message)
+	// Place into queue if window is still loading
+	if(!force && status != TGUI_WINDOW_READY)
+		if(!message_queue)
+			message_queue = list()
+		message_queue += list(message)
+		return
+	client << output(message, "[id].browser:update")
+
+/**
+ * public
+ *
+ * Makes an asset available to use in tgui.
+ *
+ * required asset datum/asset
+ */
+/datum/tgui_window/proc/send_asset(datum/asset/asset)
+	if(!client || !asset)
+		return
+	if(istype(asset, /datum/asset/spritesheet))
+		var/datum/asset/spritesheet/spritesheet = asset
+		send_message("asset/stylesheet", spritesheet.css_filename())
+	send_message("asset/mappings", asset.get_url_mappings())
+	sent_assets += list(asset)
+	asset.send(client)
+
+/**
+ * private
+ *
+ * Sends queued messages if the queue wasn't empty.
+ */
+/datum/tgui_window/proc/flush_message_queue()
+	if(!client || !message_queue)
+		return
+	for(var/message in message_queue)
+		client << output(message, "[id].browser:update")
+	message_queue = null
+
+/**
+ * private
+ *
+ * Callback for handling incoming tgui messages.
+ */
+/datum/tgui_window/proc/on_message(type, list/payload, list/href_list)
+	switch(type)
+		if("ready")
+			// Status can be READY if user has refreshed the window.
+			if(status == TGUI_WINDOW_READY)
+				// Resend the assets
+				for(var/asset in sent_assets)
+					send_asset(asset)
+			status = TGUI_WINDOW_READY
+		if("log")
+			if(href_list["fatal"])
+				fatally_errored = TRUE
+	// Pass message to UI that requested the lock
+	if(locked && locked_by)
+		locked_by.on_message(type, payload, href_list)
+		flush_message_queue()
+		return
+	// If not locked, handle these message types
+	switch(type)
+		if("suspend")
+			close(can_be_suspended = TRUE)
+		if("close")
+			close(can_be_suspended = FALSE)