1 files changed, 52 insertions, 0 deletions
diff --git a/www/wiki/resources/src/mediawiki/mediawiki.requestIdleCallback.js b/www/wiki/resources/src/mediawiki/mediawiki.requestIdleCallback.js
new file mode 100644
index 00000000..650092bd
--- /dev/null
+++ b/www/wiki/resources/src/mediawiki/mediawiki.requestIdleCallback.js
@@ -0,0 +1,52 @@
+( function ( mw ) {
+	var maxBusy = 50;
+
+	mw.requestIdleCallbackInternal = function ( callback ) {
+		setTimeout( function () {
+			var start = mw.now();
+			callback( {
+				didTimeout: false,
+				timeRemaining: function () {
+					return Math.max( 0, maxBusy - ( mw.now() - start ) );
+				}
+			} );
+		}, 1 );
+	};
+
+	/**
+	 * Schedule a deferred task to run in the background.
+	 *
+	 * This allows code to perform tasks in the main thread without impacting
+	 * time-critical operations such as animations and response to input events.
+	 *
+	 * Basic logic is as follows:
+	 *
+	 * - User input event should be acknowledged within 100ms per [RAIL].
+	 * - Idle work should be grouped in blocks of upto 50ms so that enough time
+	 *   remains for the event handler to execute and any rendering to take place.
+	 * - Whenever a native event happens (e.g. user input), the deadline for any
+	 *   running idle callback drops to 0.
+	 * - As long as the deadline is non-zero, other callbacks pending may be
+	 *   executed in the same idle period.
+	 *
+	 * See also:
+	 *
+	 * - <https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback>
+	 * - <https://w3c.github.io/requestidlecallback/>
+	 * - <https://developers.google.com/web/updates/2015/08/using-requestidlecallback>
+	 * [RAIL]: https://developers.google.com/web/fundamentals/performance/rail
+	 *
+	 * @member mw
+	 * @param {Function} callback
+	 * @param {Object} [options]
+	 * @param {number} [options.timeout] If set, the callback will be scheduled for
+	 *  immediate execution after this amount of time (in milliseconds) if it didn't run
+	 *  by that time.
+	 */
+	mw.requestIdleCallback = window.requestIdleCallback ?
+		window.requestIdleCallback.bind( window ) : // Bind because it throws TypeError if context is not window
+		mw.requestIdleCallbackInternal;
+	// Note: Polyfill was previously disabled due to
+	// https://bugs.chromium.org/p/chromium/issues/detail?id=647870
+	// See also <http://codepen.io/Krinkle/full/XNGEvv>
+}( mediaWiki ) );