Bug 1186061 patch 1 - Add feature to reftest harness to allow skipping the flush. r=mattwoodrow

layout/tools/reftest/README.txt

@@ -491,6 +491,16 @@ In a reftest-wait test, to disable testing of invalidation and force the final
 snapshot to be taken of the whole window, set the "reftest-snapshot-all"
 class on the root element.
 
+Avoid triggering flushes: class="reftest-no-flush"
+==================================================
+
+The reftest harness normally triggers flushes by calling
+getBoundingClientRect on the root element.  If the root element of the
+test has class="reftest-no-flush", it doesn't do this.
+
+This is useful for testing animations on the compositor thread, since
+the flushing will cause a main thread style update.
+
 Zoom Tests: reftest-zoom="<float>"
 ==================================
 

layout/tools/reftest/reftest-content.js

@@ -402,10 +402,11 @@ function FlushRendering() {
                     .getInterface(CI.nsIDOMWindowUtils);
         var afterPaintWasPending = utils.isMozAfterPaintPending;
 
-        if (win.document.documentElement) {
+        var root = win.document.documentElement;
+        if (root && !root.classList.contains("reftest-no-flush")) {
             try {
                 // Flush pending restyles and reflows for this window
-                win.document.documentElement.getBoundingClientRect();
+                root.getBoundingClientRect();
             } catch (e) {
                 LogWarning("flushWindow failed: " + e + "\n");
             }