Bug 802856 - Plugins don't always get focused in HiDPI mode if you click on them. r=jfkthame

widget/cocoa/nsChildView.mm

@@ -3959,10 +3959,11 @@ static int32_t RoundUp(double aDouble)
 
   nsAutoRetainCocoaObject kungFuDeathGrip(self);
 
+  // hitTest needs coordinates in device pixels
   NSPoint eventLoc = nsCocoaUtils::ScreenLocationForEvent(currentEvent);
   eventLoc.y = nsCocoaUtils::FlippedScreenY(eventLoc.y);
-  nsIntPoint widgetLoc(NSToIntRound(eventLoc.x), NSToIntRound(eventLoc.y));
-  widgetLoc -= mGeckoChild->WidgetToScreenOffset();
+  nsIntPoint widgetLoc = mGeckoChild->CocoaPointsToDevPixels(eventLoc) -
+    mGeckoChild->WidgetToScreenOffset();
 
   nsQueryContentEvent hitTest(true, NS_QUERY_DOM_WIDGET_HITTEST, mGeckoChild);
   hitTest.InitForQueryDOMWidgetHittest(widgetLoc);