Bug 1482825 - Only prevent dispatching mouse-move events targeted at a slider frame to web content, if the scrollbar is being dragged. r=smaug

Otherwise, if the mouse is just moving over the scrollbar, mouse-move
events are dispatched to web content.

Differential Revision: https://phabricator.services.mozilla.com/D4800

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

layout/xul/nsSliderFrame.cpp

@@ -1196,7 +1196,7 @@ nsSliderFrame::DragThumb(bool aGrabMouseEvents)
 }
 
 bool
-nsSliderFrame::isDraggingThumb()
+nsSliderFrame::isDraggingThumb() const
 {
   return (nsIPresShell::GetCapturingContent() == GetContent());
 }
@@ -1567,7 +1567,8 @@ nsSliderFrame::OnlySystemGroupDispatch(EventMessage aMessage) const
   // If we are in a native anonymous subtree, do not dispatch mouse-move events
   // targeted at this slider frame to web content. This matches the behaviour
   // of other browsers.
-  return aMessage == eMouseMove && GetContent()->IsInNativeAnonymousSubtree();
+  return aMessage == eMouseMove && isDraggingThumb() &&
+         GetContent()->IsInNativeAnonymousSubtree();
 }
 
 NS_IMPL_ISUPPORTS(nsSliderMediator,

layout/xul/nsSliderFrame.h

@@ -162,7 +162,7 @@ private:
   void DragThumb(bool aGrabMouseEvents);
   void AddListener();
   void RemoveListener();
-  bool isDraggingThumb();
+  bool isDraggingThumb() const;
 
   void SuppressDisplayport();
   void UnsuppressDisplayport();