Bug 1352306 - Part 2: stylo: Only snapshot EventStates if there is some rule that depends on it. r=emilio

MozReview-Commit-ID: J5xhdi7pGSv

--HG--
extra : rebase_source : 48247c1dd2e5b091224bc5fff2e1249b880e157b

layout/base/ServoRestyleManager.cpp

@@ -588,6 +588,16 @@ ServoRestyleManager::ContentStateChanged(nsIContent* aContent,
   ContentStateChangedInternal(aElement, aChangedBits, &changeHint,
                               &restyleHint);
 
+  // Don't bother taking a snapshot if no rules depend on these state bits.
+  //
+  // We always take a snapshot for the LTR/RTL event states, since Servo doesn't
+  // track those bits in the same way, and we know that :dir() rules are always
+  // present in UA style sheets.
+  if (!aChangedBits.HasAtLeastOneOfStates(DIRECTION_STATES) &&
+      !StyleSet()->HasStateDependency(aChangedBits)) {
+    return;
+  }
+
   ServoElementSnapshot& snapshot = SnapshotFor(aElement);
   EventStates previousState = aElement->StyleState() ^ aChangedBits;
   snapshot.AddState(previousState);

layout/style/ServoBindingList.h

@@ -85,6 +85,9 @@ SERVO_BINDING_FUNC(Servo_StyleSet_ResolveForDeclarations,
 SERVO_BINDING_FUNC(Servo_StyleSet_MightHaveAttributeDependency, bool,
                    RawServoStyleSetBorrowed set,
                    nsIAtom* local_name)
+SERVO_BINDING_FUNC(Servo_StyleSet_HasStateDependency, bool,
+                   RawServoStyleSetBorrowed set,
+                   uint64_t state)
 
 
 // CSSRuleList

layout/style/ServoStyleSet.cpp

@@ -1161,4 +1161,10 @@ ServoStyleSet::MightHaveAttributeDependency(nsIAtom* aAttribute)
   return Servo_StyleSet_MightHaveAttributeDependency(mRawSet.get(), aAttribute);
 }
 
+bool
+ServoStyleSet::HasStateDependency(EventStates aState)
+{
+  return Servo_StyleSet_HasStateDependency(mRawSet.get(), aState.ServoValue());
+}
+
 ServoStyleSet* ServoStyleSet::sInServoTraversal = nullptr;

layout/style/ServoStyleSet.h

@@ -349,6 +349,16 @@ public:
    */
   bool MightHaveAttributeDependency(nsIAtom* aAttribute);
 
+  /**
+   * Returns true if a change in event state on an element might require
+   * us to restyle the element.
+   *
+   * This function allows us to skip taking a state snapshot when
+   * the changed state isn't depended upon by any pseudo-class selectors
+   * in a style sheet.
+   */
+  bool HasStateDependency(EventStates aState);
+
 private:
   // On construction, sets sInServoTraversal to the given ServoStyleSet.
   // On destruction, clears sInServoTraversal and calls RunPostTraversalTasks.