Bug 1386777 - Interpret autoscroll anchor coordinates correctly when there is a device scale. r=kats

MozReview-Commit-ID: 6caXxWjuqxm

--HG--
extra : rebase_source : 4052dd05a4c05cc8fc4effe0f7911179aa8005d0

dom/interfaces/base/nsITabParent.idl

@@ -81,7 +81,7 @@ interface nsITabParent : nsISupports
   /**
    * Notify APZ to start autoscrolling.
    * (aAnchorX, aAnchorY) are the coordinates of the autoscroll anchor,
-   * in LayoutDevice coordinates relative to the screen. aScrollId and 
+   * in CSS coordinates relative to the screen. aScrollId and 
    * aPresShellId identify the scroll frame that content chose to scroll.
    */
   void startApzAutoscroll(in float aAnchorX, in float aAnchorY,

dom/ipc/TabParent.cpp

@@ -3283,7 +3283,8 @@ TabParent::StartApzAutoscroll(float aAnchorX, float aAnchorY,
       // The anchor coordinates that are passed in are relative to the origin
       // of the screen, but we are sending them to APZ which only knows about
       // coordinates relative to the widget, so convert them accordingly.
-      LayoutDeviceIntPoint anchor = RoundedToInt(LayoutDevicePoint{aAnchorX, aAnchorY});
+      CSSPoint anchorCss{aAnchorX, aAnchorY};
+      LayoutDeviceIntPoint anchor = RoundedToInt(anchorCss * widget->GetDefaultScale());
       anchor -= widget->WidgetToScreenOffset();
 
       widget->StartAsyncAutoscroll(