Bug 1137031 - Do not stop restyling when TryStartingTransitiosn gives us a new style context. r=dbaron

It is not correct to stop restyling by leaving the old style context on
the frame and returning eRestyleResult_Stop when TryStartingTransition
provides a new, without-animations style context.  This is at least
because the new new style context could have different styles from the
old new style context.

layout/base/RestyleManager.cpp

@@ -2015,7 +2015,7 @@ RestyleManager::DebugVerifyStyleTree(nsIFrame* aFrame)
 
 // aContent must be the content for the frame in question, which may be
 // :before/:after content
-/* static */ void
+/* static */ bool
 RestyleManager::TryStartingTransition(nsPresContext* aPresContext,
                                       nsIContent* aContent,
                                       nsStyleContext* aOldStyleContext,
@@ -2023,13 +2023,15 @@ RestyleManager::TryStartingTransition(nsPresContext* aPresContext,
                                         aNewStyleContext /* inout */)
 {
   if (!aContent || !aContent->IsElement()) {
-    return;
+    return false;
   }
 
   // Notify the transition manager.  If it starts a transition,
   // it might modify the new style context.
+  nsRefPtr<nsStyleContext> sc = *aNewStyleContext;
   aPresContext->TransitionManager()->StyleContextChanged(
     aContent->AsElement(), aOldStyleContext, aNewStyleContext);
+  return *aNewStyleContext != sc;
 }
 
 static dom::Element*
@@ -3300,9 +3302,13 @@ ElementRestyler::RestyleSelf(nsIFrame* aSelf,
         }
       }
     } else {
-      RestyleManager::TryStartingTransition(mPresContext, aSelf->GetContent(),
-                                            oldContext, &newContext);
-
+      bool changedStyle =
+        RestyleManager::TryStartingTransition(mPresContext, aSelf->GetContent(),
+                                              oldContext, &newContext);
+      if (changedStyle) {
+        LOG_RESTYLE_CONTINUE("TryStartingTransition changed the new style context");
+        result = eRestyleResult_Continue;
+      }
       CaptureChange(oldContext, newContext, assumeDifferenceHint,
                     &equalStructs);
       if (equalStructs != NS_STYLE_INHERIT_MASK) {

layout/base/RestyleManager.h

@@ -228,12 +228,13 @@ public:
   /**
    * Try starting a transition for an element or a ::before or ::after
    * pseudo-element, given an old and new style context.  This may
-   * change the new style context if a transition is started.
+   * change the new style context if a transition is started.  Returns
+   * true iff it does change aNewStyleContext.
    *
    * For the pseudo-elements, aContent must be the anonymous content
    * that we're creating for that pseudo-element, not the real element.
    */
-  static void
+  static bool
   TryStartingTransition(nsPresContext* aPresContext, nsIContent* aContent,
                         nsStyleContext* aOldStyleContext,
                         nsRefPtr<nsStyleContext>* aNewStyleContext /* inout */);