Bug 1131695 - Extract a GetLayoutDeviceToCSSScale helper function in TabParent. r=fabrice

dom/ipc/TabParent.cpp

@@ -1160,23 +1160,21 @@ bool TabParent::SendRealMouseEvent(WidgetMouseEvent& event)
   return SendRealMouseButtonEvent(event);
 }
 
-CSSPoint TabParent::AdjustTapToChildWidget(const CSSPoint& aPoint)
+LayoutDeviceToCSSScale
+TabParent::GetLayoutDeviceToCSSScale()
 {
   nsCOMPtr<nsIContent> content = do_QueryInterface(mFrameElement);
+  nsIDocument* doc = (content ? content->OwnerDoc() : nullptr);
+  nsIPresShell* shell = (doc ? doc->GetShell() : nullptr);
+  nsPresContext* ctx = (shell ? shell->GetPresContext() : nullptr);
+  return LayoutDeviceToCSSScale(ctx
+    ? (float)ctx->AppUnitsPerDevPixel() / nsPresContext::AppUnitsPerCSSPixel()
+    : 0.0f);
+}
 
-  if (!content || !content->OwnerDoc()) {
-    return aPoint;
-  }
-
-  nsIDocument* doc = content->OwnerDoc();
-  if (!doc || !doc->GetShell()) {
-    return aPoint;
-  }
-  nsPresContext* presContext = doc->GetShell()->GetPresContext();
-
-  return aPoint + CSSPoint(
-    presContext->DevPixelsToFloatCSSPixels(mChildProcessOffsetAtTouchStart.x),
-    presContext->DevPixelsToFloatCSSPixels(mChildProcessOffsetAtTouchStart.y));
+CSSPoint TabParent::AdjustTapToChildWidget(const CSSPoint& aPoint)
+{
+  return aPoint + (LayoutDevicePoint(mChildProcessOffsetAtTouchStart) * GetLayoutDeviceToCSSScale());
 }
 
 bool TabParent::SendHandleSingleTap(const CSSPoint& aPoint, const ScrollableLayerGuid& aGuid)

dom/ipc/TabParent.h

@@ -264,6 +264,7 @@ public:
     bool MapEventCoordinatesForChildProcess(mozilla::WidgetEvent* aEvent);
     void MapEventCoordinatesForChildProcess(const LayoutDeviceIntPoint& aOffset,
                                             mozilla::WidgetEvent* aEvent);
+    LayoutDeviceToCSSScale GetLayoutDeviceToCSSScale();
 
     virtual bool RecvRequestNativeKeyBindings(const mozilla::WidgetKeyboardEvent& aEvent,
                                               MaybeNativeKeyBinding* aBindings) MOZ_OVERRIDE;