Bug 1505880 - set mGestureDownPoint when receiving 'touchstart' event. r=smaug

We should set the 'mGestureDownPoint' correctly when receiving 'touchstart' so that we can
distinguish whether the 'touchend' comes from dragging or simply touching by calling
'IsEventOutsideDragThreshold()'.

Differential Revision: https://phabricator.services.mozilla.com/D11395

--HG--
extra : moz-landing-system : lando

dom/events/EventStateManager.cpp

@@ -844,6 +844,9 @@ EventStateManager::PreHandleEvent(nsPresContext* aPresContext,
       compositionEvent->mData = selectedText.mReply.mString;
     }
     break;
+  case eTouchStart:
+    SetGestureDownPoint(aEvent->AsTouchEvent());
+    break;
   case eTouchEnd:
     NotifyTargetUserActivation(aEvent, aTargetContent);
     break;
@@ -1688,8 +1691,7 @@ EventStateManager::BeginTrackingDragGesture(nsPresContext* aPresContext,
 
   // Note that |inDownEvent| could be either a mouse down event or a
   // synthesized mouse move event.
-  mGestureDownPoint =
-    inDownEvent->mRefPoint + inDownEvent->mWidget->WidgetToScreenOffset();
+  SetGestureDownPoint(inDownEvent);
 
   if (inDownFrame) {
     inDownFrame->GetContentForEvent(inDownEvent,
@@ -1709,6 +1711,21 @@ EventStateManager::BeginTrackingDragGesture(nsPresContext* aPresContext,
   }
 }
 
+void
+EventStateManager::SetGestureDownPoint(WidgetGUIEvent* aEvent)
+{
+  mGestureDownPoint =
+    GetEventRefPoint(aEvent) + aEvent->mWidget->WidgetToScreenOffset();
+}
+
+LayoutDeviceIntPoint
+EventStateManager::GetEventRefPoint(WidgetEvent* aEvent) const
+{
+  auto touchEvent = aEvent->AsTouchEvent();
+  return (touchEvent && !touchEvent->mTouches.IsEmpty()) ?
+    aEvent->AsTouchEvent()->mTouches[0]->mRefPoint : aEvent->mRefPoint;
+}
+
 void
 EventStateManager::BeginTrackingRemoteDragGesture(nsIContent* aContent)
 {
@@ -1808,11 +1825,8 @@ EventStateManager::IsEventOutsideDragThreshold(WidgetInputEvent* aEvent) const
       sPixelThresholdY = 5;
   }
 
-  auto touchEvent = aEvent->AsTouchEvent();
-  LayoutDeviceIntPoint pt = aEvent->mWidget->WidgetToScreenOffset() +
-    ((touchEvent && !touchEvent->mTouches.IsEmpty())
-      ? aEvent->AsTouchEvent()->mTouches[0]->mRefPoint
-      : aEvent->mRefPoint);
+  LayoutDeviceIntPoint pt =
+    aEvent->mWidget->WidgetToScreenOffset() + GetEventRefPoint(aEvent);
   LayoutDeviceIntPoint distance = pt - mGestureDownPoint;
   return
     Abs(distance.x) > AssertedCast<uint32_t>(sPixelThresholdX) ||

dom/events/EventStateManager.h

@@ -1096,6 +1096,10 @@ protected:
                                 WidgetMouseEvent* aDownEvent,
                                 nsIFrame* aDownFrame);
 
+  void SetGestureDownPoint(WidgetGUIEvent* aEvent);
+
+  LayoutDeviceIntPoint GetEventRefPoint(WidgetEvent* aEvent) const;
+
   friend class mozilla::dom::TabParent;
   void BeginTrackingRemoteDragGesture(nsIContent* aContent);
   void StopTrackingDragGesture();