Fixes Goonchat scrolling when pruning messages

The scroll position is now adjusted to compensate for the elements that
get removed. To cut down on the subpixel drift caused by their removal,
message pruning is now only done every 2 seconds.

goon/browserassets/js/browserOutput.js

@@ -215,13 +215,7 @@ function output(message, flag) {
 		message = linkify(message);
 	}
 
-	opts.messageCount++;
-
-	//Pop the top message off if history limit reached
-	if (opts.messageCount >= opts.messageLimit) {
-		$messages.children('div.entry:first-child').remove();
-		opts.messageCount--; //I guess the count should only ever equal the limit
-	}
+	opts.messageCount++;	
 
 	//Actually append the message
 	var entry = document.createElement('div');
@@ -463,6 +457,20 @@ $(function() {
 			$('.connectionClosed[data-count="'+opts.noResponseCount+'"]:not(.restored)').addClass('restored').text('Your connection has been restored (probably)!');
 			opts.noResponse = false;
 		}
+		if (opts.messageCount > opts.messageLimit) { // Prune old messages beyond the message limit
+			var bodyHeight = $('body').height();
+			var messagesHeight = $messages.outerHeight();
+			var scrollPos = $(window).scrollTop();
+			var atBottom = (bodyHeight + scrollPos >= messagesHeight - opts.scrollSnapTolerance)
+
+			$messages.children().slice(0, opts.messageCount - opts.messageLimit).remove();
+			opts.messageCount = opts.messageLimit;
+			if (!atBottom) {
+				// If we weren't at the bottom, adjust scroll position to compensate for removed elements
+				var newPos = scrollPos - (messagesHeight - $messages.outerHeight())
+				$('body,html').scrollTop(newPos);
+			}
+		}
 	}, 2000); //2 seconds