Bug 1514501 - Fix screen selection and scaling for fullscreen r=jmathies

I needed to scale the layout device pixels coming from Windows before passing them to ScreenForRect(). Also, I'm using GetRect() directly instead of GetRectDisplayPix() * scale now, to avoid an unnecessary double scale & round which was making fullscreen windows off by one pixel in many cases.

Differential Revision: https://phabricator.services.mozilla.com/D15604

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

widget/windows/nsWindow.cpp

@@ -6500,19 +6500,22 @@ void nsWindow::OnWindowPosChanging(LPWINDOWPOS& info) {
     nsCOMPtr<nsIScreenManager> screenmgr =
         do_GetService(sScreenManagerContractID);
     if (screenmgr) {
+      LayoutDeviceIntRect bounds(info->x, info->y, info->cx, info->cy);
+      DesktopIntRect deskBounds =
+          RoundedToInt(bounds / GetDesktopToDeviceScale());
       nsCOMPtr<nsIScreen> screen;
-      screenmgr->ScreenForRect(info->x, info->y, info->cx, info->cy,
+      screenmgr->ScreenForRect(deskBounds.X(), deskBounds.Y(),
+                               deskBounds.Width(), deskBounds.Height(),
                                getter_AddRefs(screen));
 
       if (screen) {
         int32_t x, y, width, height;
-        screen->GetRectDisplayPix(&x, &y, &width, &height);
-        double scale = GetDesktopToDeviceScale().scale;
+        screen->GetRect(&x, &y, &width, &height);
 
-        info->x = NSToIntRound(x * scale);
-        info->y = NSToIntRound(y * scale);
-        info->cx = NSToIntRound(width * scale);
-        info->cy = NSToIntRound(height * scale);
+        info->x = x;
+        info->y = y;
+        info->cx = width;
+        info->cy = height;
       }
     }
   }