Bug 1425573 - Require target confirmation for events that hit unlayerized scroll frames with non-default overscroll-behavior. r=kats

MozReview-Commit-ID: K88tXahKg8H

--HG--
extra : rebase_source : 5edc6d20150feac46ce1dd70030c677520b41ace

gfx/layers/apz/src/APZUtils.h

@@ -80,14 +80,17 @@ CompleteAsyncTransform(const AsyncTransformComponentMatrix& aMatrix)
 struct TargetConfirmationFlags {
   explicit TargetConfirmationFlags(bool aTargetConfirmed)
     : mTargetConfirmed(aTargetConfirmed)
+    , mRequiresTargetConfirmation(false)
   {}
 
   explicit TargetConfirmationFlags(gfx::CompositorHitTestInfo aHitTestInfo)
     : mTargetConfirmed(aHitTestInfo != gfx::CompositorHitTestInfo::eInvisibleToHitTest &&
                        !(aHitTestInfo & gfx::CompositorHitTestInfo::eDispatchToContent))
+    , mRequiresTargetConfirmation(aHitTestInfo & gfx::CompositorHitTestInfo::eRequiresTargetConfirmation)
   {}
 
   bool mTargetConfirmed : 1;
+  bool mRequiresTargetConfirmation : 1;
 };
 
 } // namespace layers

gfx/layers/apz/src/InputBlockState.cpp

@@ -27,6 +27,7 @@ InputBlockState::InputBlockState(const RefPtr<AsyncPanZoomController>& aTargetAp
   : mTargetApzc(aTargetApzc)
   , mTargetConfirmed(aFlags.mTargetConfirmed ? TargetConfirmationState::eConfirmed
                                              : TargetConfirmationState::eUnconfirmed)
+  , mRequiresTargetConfirmation(aFlags.mRequiresTargetConfirmation)
   , mBlockId(sBlockCounter++)
   , mTransformToApzc(aTargetApzc->GetTransformToThis())
 {
@@ -111,7 +112,7 @@ InputBlockState::HasReceivedRealConfirmedTarget() const
 bool
 InputBlockState::ShouldDropEvents() const
 {
-  return false;
+  return mRequiresTargetConfirmation && (mTargetConfirmed != TargetConfirmationState::eConfirmed);
 }
 
 bool

gfx/layers/apz/src/InputBlockState.h

@@ -112,6 +112,7 @@ private:
 private:
   RefPtr<AsyncPanZoomController> mTargetApzc;
   TargetConfirmationState mTargetConfirmed;
+  bool mRequiresTargetConfirmation;
   const uint64_t mBlockId;
 
   // The APZC that was actually scrolled by events in this input block.

gfx/src/CompositorHitTestInfo.h

@@ -46,9 +46,15 @@ enum class CompositorHitTestInfo : uint16_t {
   // one (if set) or a horizontal one (if not set)
   eScrollbarVertical = 1 << 8,
 
+  // Events targeting this frame should only be processed if a target
+  // confirmation is received from the main thread. If no such confirmation
+  // is received within a timeout period, the event may be dropped.
+  // Only meaningful in combination with eDispatchToContent.
+  eRequiresTargetConfirmation = 1 << 9,
+
   // Used for IPDL serialization. This bitmask should include all the bits
   // that are defined in the enum.
-  ALL_BITS = (1 << 9) - 1,
+  ALL_BITS = (1 << 10) - 1,
 };
 
 MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(CompositorHitTestInfo)

layout/generic/nsGfxScrollFrame.cpp

@@ -3683,6 +3683,17 @@ ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
       if (aBuilder->BuildCompositorHitTestInfo()) {
         CompositorHitTestInfo info = CompositorHitTestInfo::eVisibleToHitTest
                                    | CompositorHitTestInfo::eDispatchToContent;
+        // If the scroll frame has non-default overscroll-behavior, instruct
+        // APZ to require a target confirmation before processing events that
+        // hit this scroll frame (that is, to drop the events if a confirmation
+        // does not arrive within the timeout period). Otherwise, APZ's
+        // fallback behaviour of scrolling the enclosing scroll frame would
+        // violate the specified overscroll-behavior.
+        ScrollbarStyles scrollbarStyles = GetScrollbarStylesFromFrame();
+        if (scrollbarStyles.mOverscrollBehaviorX != StyleOverscrollBehavior::Auto ||
+            scrollbarStyles.mOverscrollBehaviorY != StyleOverscrollBehavior::Auto) {
+          info |= CompositorHitTestInfo::eRequiresTargetConfirmation;
+        }
         nsDisplayCompositorHitTestInfo* hitInfo =
             MakeDisplayItem<nsDisplayCompositorHitTestInfo>(aBuilder, mScrolledFrame, info, 1,
                 Some(mScrollPort + aBuilder->ToReferenceFrame(mOuter)));