From aed23b285701dbfe461d69420b2d93c06db74dfa Mon Sep 17 00:00:00 2001
From: Kartikaya Gupta <kgupta@mozilla.com>
Date: Tue, 3 Nov 2015 14:21:40 -0500
Subject: [PATCH] Bug 1141884 - Handle wheel events on the main thread if the
 frame has snapping. r=dvander,mstange

---
 dom/events/EventStateManager.cpp |  8 ++++++++
 layout/base/nsDisplayList.cpp    |  6 ++++++
 layout/base/nsLayoutUtils.cpp    | 12 ++++++++++++
 layout/base/nsLayoutUtils.h      |  6 ++++++
 4 files changed, 32 insertions(+)

diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index c35493b67fd..c9ea91a6372 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -3119,6 +3119,14 @@ EventStateManager::PostHandleEvent(nsPresContext* aPresContext,
       if (pluginFrame) {
         MOZ_ASSERT(pluginFrame->WantsToHandleWheelEventAsDefaultAction());
         action = WheelPrefs::ACTION_SEND_TO_PLUGIN;
+      } else if (nsLayoutUtils::IsScrollFrameWithSnapping(frameToScroll)) {
+        // If the target has scroll-snapping points then we want to handle
+        // the wheel event on the main thread even if we have APZ enabled. Do
+        // so and let the APZ know that it should ignore this event.
+        if (wheelEvent->mFlags.mHandledByAPZ) {
+          wheelEvent->mFlags.mDefaultPrevented = true;
+        }
+        action = WheelPrefs::GetInstance()->ComputeActionFor(wheelEvent);
       } else if (wheelEvent->mFlags.mHandledByAPZ) {
         action = WheelPrefs::ACTION_NONE;
       } else {
diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index e8bd59fdb49..9250d8fba21 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -3281,6 +3281,12 @@ nsDisplayLayerEventRegions::AddFrame(nsDisplayListBuilder* aBuilder,
     if (pluginFrame && pluginFrame->WantsToHandleWheelEventAsDefaultAction()) {
       mDispatchToContentHitRegion.Or(mDispatchToContentHitRegion, borderBox);
     }
+  } else if (gfxPlatform::GetPlatform()->SupportsApzWheelInput() &&
+             nsLayoutUtils::IsScrollFrameWithSnapping(aFrame->GetParent())) {
+    // If the frame is the inner content of a scrollable frame with snap-points
+    // then we want to handle wheel events for it on the main thread. Add it to
+    // the d-t-c region so that APZ waits for the main thread.
+    mDispatchToContentHitRegion.Or(mDispatchToContentHitRegion, borderBox);
   }
 
   // Touch action region
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 007e7867730..8b1255a1f52 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -8676,3 +8676,15 @@ nsLayoutUtils::GetSelectionBoundingRect(Selection* aSel)
 
   return res;
 }
+
+/* static */ bool
+nsLayoutUtils::IsScrollFrameWithSnapping(nsIFrame* aFrame)
+{
+  nsIScrollableFrame* sf = do_QueryFrame(aFrame);
+  if (!sf) {
+    return false;
+  }
+  ScrollbarStyles styles = sf->GetScrollbarStyles();
+  return styles.mScrollSnapTypeY != NS_STYLE_SCROLL_SNAP_TYPE_NONE ||
+         styles.mScrollSnapTypeX != NS_STYLE_SCROLL_SNAP_TYPE_NONE;
+}
diff --git a/layout/base/nsLayoutUtils.h b/layout/base/nsLayoutUtils.h
index 9b2ce653025..3b4416dda69 100644
--- a/layout/base/nsLayoutUtils.h
+++ b/layout/base/nsLayoutUtils.h
@@ -2773,6 +2773,12 @@ public:
    * @param aSel      Selection to check
    */
   static nsRect GetSelectionBoundingRect(mozilla::dom::Selection* aSel);
+
+  /**
+   * Returns true if the given frame is a scrollframe and it has snap points.
+   */
+  static bool IsScrollFrameWithSnapping(nsIFrame* aFrame);
+
 private:
   static uint32_t sFontSizeInflationEmPerLine;
   static uint32_t sFontSizeInflationMinTwips;