Bug 1702097 - Reset NSAppearance.currentAppearance before native theme drawing. r=spohl

This is global state, so it can have remnants from random other state changes.
Starting with Big Sur, currentAppearance is deprecated in favor of
performAsCurrentDrawingAppearance, which really does seem like a better API for
this.

Differential Revision: https://phabricator.services.mozilla.com/D110361

widget/cocoa/nsNativeThemeCocoa.mm

@@ -2646,8 +2646,7 @@ Maybe<nsNativeThemeCocoa::WidgetInfo> nsNativeThemeCocoa::ComputeWidgetInfo(
 NS_IMETHODIMP
 nsNativeThemeCocoa::DrawWidgetBackground(gfxContext* aContext, nsIFrame* aFrame,
                                          StyleAppearance aAppearance, const nsRect& aRect,
-                                         const nsRect& aDirtyRect,
-                                         DrawOverflow) {
+                                         const nsRect& aDirtyRect, DrawOverflow) {
   NS_OBJC_BEGIN_TRY_BLOCK_RETURN;
 
   Maybe<WidgetInfo> widgetInfo = ComputeWidgetInfo(aFrame, aAppearance, aRect);
@@ -2674,6 +2673,16 @@ nsNativeThemeCocoa::DrawWidgetBackground(gfxContext* aContext, nsIFrame* aFrame,
 void nsNativeThemeCocoa::RenderWidget(const WidgetInfo& aWidgetInfo, DrawTarget& aDrawTarget,
                                       const gfx::Rect& aWidgetRect, const gfx::Rect& aDirtyRect,
                                       float aScale) {
+  if (@available(macOS 10.14, *)) {
+    // Some of the drawing below uses NSAppearance.currentAppearance behind the scenes.
+    // Set it to the appearance we want, to overwrite any state that's still around from earlier
+    // paints. We set it to NSApp.effectiveAppearance, which is consistent with what nsLookAndFeel
+    // uses for CSS system colors; it's either the system appearance or our Aqua override.
+    // We could also make nsNativeThemeCocoa respect the per-window appearance, but only once CSS
+    // system colors also respect the window appearance.
+    NSAppearance.currentAppearance = NSApp.effectiveAppearance;
+  }
+
   const Widget widget = aWidgetInfo.Widget();
 
   // Some widgets render using DrawTarget, and some using CGContext.