Bug 1645782 - Correctly snap windows custom scrollbar track / thumbs. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D97150

widget/windows/nsNativeThemeWin.cpp

@@ -2596,7 +2596,7 @@ nsITheme::Transparency nsNativeThemeWin::GetWidgetTransparency(
           ui->mScrollbarColor.AsColors().track.MaybeTransparent()) {
         return eTransparent;
       }
-      // DrawCustomScrollbarPart doesn't draw the track background for
+      // MayDrawCustomScrollbarPart doesn't draw the track background for
       // widgets on it, and these widgets are thinner than the track,
       // so we need to return transparent for them.
       switch (aAppearance) {
@@ -4042,12 +4042,11 @@ bool nsNativeThemeWin::MayDrawCustomScrollbarPart(gfxContext* aContext,
 
   gfxContextAutoSaveRestore autoSave(aContext);
   RefPtr<gfxContext> ctx = aContext;
+  DrawTarget* dt = ctx->GetDrawTarget();
   gfxFloat p2a = gfxFloat(aFrame->PresContext()->AppUnitsPerDevPixel());
-  gfxRect clipRect = ThebesRect(
-      LayoutDevicePixel::FromAppUnits(aClipRect, p2a).ToUnknownRect());
+  gfxRect rect = ThebesRect(NSRectToSnappedRect(aRect, p2a, *dt));
+  gfxRect clipRect = ThebesRect(NSRectToSnappedRect(aClipRect, p2a, *dt));
   ctx->Clip(clipRect);
-  gfxRect rect =
-      ThebesRect(LayoutDevicePixel::FromAppUnits(aRect, p2a).ToUnknownRect());
 
   const nsStyleUI* ui = style->StyleUI();
   auto* customColors =