From 0d7ef032b71e9eaef02489e70f443f9776b2bebf Mon Sep 17 00:00:00 2001
From: Timo Tijhof <krinklemail@gmail.com>
Date: Thu, 19 Nov 2015 21:44:36 +0000
Subject: [PATCH] resourceloader: Don't create redundant timers in
 addEmbeddedCSS()

There's is typically two ways one might implement an eventloop-based
timeout buffer in JavaScript.

Note that "tick" refers to an iteration of the eventloop. Each iteration starts
after all other sychronous code has finished in the current script execution.

1. Early-bird
   The first call to 'addBuffer' schedules a flush in the next tick.
   Any subsequent call (before the next tick) keeps adding to same buffer.
   As scheduled, in the next 'tick' all buffered items will be flushed.

   The next call to 'addBuffer' will a new cycle again.
   Buffered items are flushed within the next tick.

2. Idle delay
   Calls to 'addBuffer' always schedule a new flush. One cancels any existing
   flush that may've been scheduled. This effectively keeps pushing the flush
   to a future tick until a tick does not buffer new items.
   Buffered items may remain in buffer for a long time.

For the CSS buffer in ResourceLoader we want the first model. And while the
implementation already behaves like that, it was creating new timers in each
call regardless. This resulted in 10 buffered items creating 10 timers.
The first timer flushes the buffer. And the remaining 9 timers all try to
flush an empty buffer.

Change-Id: I49c64fedb0ad0362e91bb782fecf3cdda7dc61fc
---
 resources/src/mediawiki/mediawiki.js | 50 ++++++++++++----------------
 1 file changed, 22 insertions(+), 28 deletions(-)

diff --git a/resources/src/mediawiki/mediawiki.js b/resources/src/mediawiki/mediawiki.js
index b58ce493d7..293fd589a4 100644
--- a/resources/src/mediawiki/mediawiki.js
+++ b/resources/src/mediawiki/mediawiki.js
@@ -800,10 +800,9 @@
 				// Selector cache for the marker element. Use getMarker() to get/use the marker!
 				$marker = null,
 
-				// Buffer for #addEmbeddedCSS
+				// For #addEmbeddedCSS
 				cssBuffer = '',
-
-				// Callbacks for #addEmbeddedCSS
+				cssBufferTimer = null,
 				cssCallbacks = $.Callbacks();
 
 			function getMarker() {
@@ -872,40 +871,35 @@
 					cssCallbacks.add( callback );
 				}
 
-				// Yield once before inserting the <style> tag. There are likely
-				// more calls coming up which we can combine this way.
-				// Appending a stylesheet and waiting for the browser to repaint
-				// is fairly expensive, this reduces that (bug 45810)
+				// Yield once before creating the <style> tag. This lets multiple stylesheets
+				// accumulate into one buffer, allowing us to reduce how often new stylesheets
+				// are inserted in the browser. Appending a stylesheet and waiting for the
+				// browser to repaint is fairly expensive. (T47810)
 				if ( cssText ) {
-					// Be careful not to extend the buffer with css that needs a new stylesheet.
-					// cssText containing `@import` rules needs to go at the start of a buffer,
-					// since those only work when placed at the start of a stylesheet; bug 35562.
+					// Don't extend the buffer if the item needs its own stylesheet.
+					// Keywords like `@import` are only valid at the start of a stylesheet (T37562).
 					if ( !cssBuffer || cssText.slice( 0, '@import'.length ) !== '@import' ) {
 						// Linebreak for somewhat distinguishable sections
-						// (the rl-cachekey comment separating each)
 						cssBuffer += '\n' + cssText;
-						// TODO: Use requestAnimationFrame in the future which will
-						// perform even better by not injecting styles while the browser
-						// is painting.
-						setTimeout( function () {
-							// Can't pass addEmbeddedCSS to setTimeout directly because Firefox
-							// (below version 13) has the non-standard behaviour of passing a
-							// numerical "lateness" value as first argument to this callback
-							// http://benalman.com/news/2009/07/the-mysterious-firefox-settime/
-							addEmbeddedCSS();
-						} );
+						// TODO: Using requestAnimationFrame would perform better by not injecting
+						// styles while the browser is busy painting.
+						if ( !cssBufferTimer ) {
+							cssBufferTimer = setTimeout( function () {
+								// Support: Firefox < 13
+								// Firefox 12 has non-standard behaviour of passing a number
+								// as first argument to a setTimeout callback.
+								// http://benalman.com/news/2009/07/the-mysterious-firefox-settime/
+								addEmbeddedCSS();
+							} );
+						}
 						return;
 					}
 
-				// This is a delayed call and we got a buffer still
-				} else if ( cssBuffer ) {
+				// This is a scheduled flush for the buffer
+				} else {
+					cssBufferTimer = null;
 					cssText = cssBuffer;
 					cssBuffer = '';
-
-				} else {
-					// This is a delayed call, but buffer was already cleared by
-					// another delayed call.
-					return;
 				}
 
 				// By default, always create a new <style>. Appending text to a <style>
-- 
2.20.1