Bug 1374966 - Drop nsChangeHint_UpdateTransformLayer when we also have nsChangeHint_AddOrRemoveTransform; r=heycam

MozReview-Commit-ID: 2eFl7cDXNA

--HG--
extra : rebase_source : 21d44a91b1848f630500495788fb704ef260333e

layout/base/RestyleManager.cpp

@@ -1531,6 +1531,25 @@ RestyleManager::ProcessRestyledFrames(nsStyleChangeList& aChangeList)
         hint &= ~nsChangeHint_UpdatePostTransformOverflow;
       }
 
+      if (hint & nsChangeHint_AddOrRemoveTransform) {
+        // When dropping a running transform animation we will first add an
+        // nsChangeHint_UpdateTransformLayer hint as part of the animation-only
+        // restyle. During the subsequent regular restyle, if the animation was
+        // the only reason the element had any transform applied, we will add
+        // nsChangeHint_AddOrRemoveTransform as part of the regular restyle.
+        //
+        // With the Gecko backend, these two change hints are processed
+        // after each restyle but when using the Servo backend they accumulate
+        // and are processed together after we have already removed the
+        // transform as part of the regular restyle. Since we don't actually
+        // need the nsChangeHint_UpdateTransformLayer hint if we already have
+        // a nsChangeHint_AddOrRemoveTransform hint, and since we
+        // will fail an assertion in ApplyRenderingChangeToTree if we try
+        // specify nsChangeHint_UpdateTransformLayer but don't have any
+        // transform style, we just drop the unneeded hint here.
+        hint &= ~nsChangeHint_UpdateTransformLayer;
+      }
+
       if (hint & nsChangeHint_UpdateEffects) {
         for (nsIFrame* cont = frame; cont;
              cont = nsLayoutUtils::GetNextContinuationOrIBSplitSibling(cont)) {