Bug 1351535 - Part 4: Add a TraversalRestyleBehavior argument to traversal functions. r=bholley

This argument will be used to control whether we are restyling in preparation
for reframing a subtree, which can avoid generating any change hints, as we
aren't preserving the frames that they would otherwise apply to.

MozReview-Commit-ID: DkLVCUnNGt

--HG--
extra : rebase_source : cb3537cea26cb9805b2ec1556cf5ca6eb9d38ab8

layout/style/ServoBindingList.h

@@ -324,7 +324,8 @@ SERVO_BINDING_FUNC(Servo_ResolveStyleLazily, ServoComputedValuesStrong,
 // directly
 SERVO_BINDING_FUNC(Servo_TraverseSubtree, bool,
                    RawGeckoElementBorrowed root, RawServoStyleSetBorrowed set,
-                   mozilla::TraversalRootBehavior root_behavior)
+                   mozilla::TraversalRootBehavior root_behavior,
+                   mozilla::TraversalRestyleBehavior restyle_behavior)
 
 // Assert that the tree has no pending or unconsumed restyles.
 SERVO_BINDING_FUNC(Servo_AssertTreeIsClean, void, RawGeckoElementBorrowed root)

layout/style/ServoStyleSet.cpp

@@ -225,7 +225,9 @@ ServoStyleSet::PreTraverse()
 
 bool
 ServoStyleSet::PrepareAndTraverseSubtree(RawGeckoElementBorrowed aRoot,
-                                         mozilla::TraversalRootBehavior aRootBehavior)
+                                         TraversalRootBehavior aRootBehavior,
+                                         TraversalRestyleBehavior
+                                           aRestyleBehavior)
 {
   // Get the Document's root element to ensure that the cache is valid before
   // calling into the (potentially-parallel) Servo traversal, where a cache hit
@@ -237,13 +239,14 @@ ServoStyleSet::PrepareAndTraverseSubtree(RawGeckoElementBorrowed aRoot,
 
   bool isInitial = !aRoot->HasServoData();
   bool postTraversalRequired =
-    Servo_TraverseSubtree(aRoot, mRawSet.get(), aRootBehavior);
+    Servo_TraverseSubtree(aRoot, mRawSet.get(), aRootBehavior, aRestyleBehavior);
   MOZ_ASSERT_IF(isInitial, !postTraversalRequired);
 
   // If there are still animation restyles needed, trigger a second traversal to
   // update CSS animations' styles.
   if (mPresContext->EffectCompositor()->PreTraverse()) {
-    if (Servo_TraverseSubtree(aRoot, mRawSet.get(), aRootBehavior)) {
+    if (Servo_TraverseSubtree(aRoot, mRawSet.get(),
+                              aRootBehavior, aRestyleBehavior)) {
       if (isInitial) {
         // We're doing initial styling, and the additional animation
         // traversal changed the styles that were set by the first traversal.
@@ -708,7 +711,9 @@ ServoStyleSet::StyleDocument()
   bool postTraversalRequired = false;
   DocumentStyleRootIterator iter(mPresContext->Document());
   while (Element* root = iter.GetNextStyleRoot()) {
-    if (PrepareAndTraverseSubtree(root, TraversalRootBehavior::Normal)) {
+    if (PrepareAndTraverseSubtree(root,
+                                  TraversalRootBehavior::Normal,
+                                  TraversalRestyleBehavior::Normal)) {
       postTraversalRequired = true;
     }
   }
@@ -723,7 +728,9 @@ ServoStyleSet::StyleNewSubtree(Element* aRoot)
   PreTraverse();
 
   DebugOnly<bool> postTraversalRequired =
-    PrepareAndTraverseSubtree(aRoot, TraversalRootBehavior::Normal);
+    PrepareAndTraverseSubtree(aRoot,
+                              TraversalRootBehavior::Normal,
+                              TraversalRestyleBehavior::Normal);
   MOZ_ASSERT(!postTraversalRequired);
 }
 
@@ -732,7 +739,9 @@ ServoStyleSet::StyleNewChildren(Element* aParent)
 {
   PreTraverse();
 
-  PrepareAndTraverseSubtree(aParent, TraversalRootBehavior::UnstyledChildrenOnly);
+  PrepareAndTraverseSubtree(aParent,
+                            TraversalRootBehavior::UnstyledChildrenOnly,
+                            TraversalRestyleBehavior::Normal);
   // We can't assert that Servo_TraverseSubtree returns false, since aParent
   // or some of its other children might have pending restyles.
 }

layout/style/ServoStyleSet.h

@@ -302,7 +302,8 @@ private:
    * a subtree.  Returns whether a post-traversal is required.
    */
   bool PrepareAndTraverseSubtree(RawGeckoElementBorrowed aRoot,
-                                 mozilla::TraversalRootBehavior aRootBehavior);
+                                 TraversalRootBehavior aRootBehavior,
+                                 TraversalRestyleBehavior aRestyleBehavior);
 
   /**
    * Clear our cached mNonInheritingStyleContexts.  We do this when we want to

layout/style/ServoTypes.h

@@ -53,6 +53,16 @@ enum class TraversalRootBehavior {
   UnstyledChildrenOnly,
 };
 
+// Indicates whether the Servo style system should perform normal processing or
+// whether it should traverse in a mode that doesn't generate any change hints,
+// which is what's required when handling frame reconstruction.  The change
+// hints in this case are unneeded, since the old frames have already been
+// destroyed.
+enum class TraversalRestyleBehavior {
+  Normal,
+  ForReconstruct,
+};
+
 // Represents which tasks are performed in a SequentialTask of UpdateAnimations.
 enum class UpdateAnimationsTasks : uint8_t {
   CSSAnimations    = 1 << 0,