Bug 1592739 - Make -moz-appearance: dialog render nothing. r=spohl

The window background will be contributed by the widget itself, which renders
them underneath Gecko's rendering.
As a result, -moz-appearance: dialog is now equivalent to the combination
-moz-appearance: none; background-color: transparent.

This change does not turn the widget itself transparent because nsCocoaWindow
does not allow top level windows to become transparent (ever since bug 1162649).
If we ever add support for top level widgets with transparent backgrounds again,
we will probably want to treat -moz-appearance: dialog differently from
-moz-appearance: none; background-color: transparent, but for now this is fine.

This change means that Gecko's rendering will go into transparent buffers. This
may result in some loss of subpixel AA in various cases.
In the main browser window, there are CSS backgound colors that cover all the
non-vibrant areas of the window, so in that window we still render mostly onto
opaque pixels.

Differential Revision: https://phabricator.services.mozilla.com/D51461

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

widget/cocoa/nsNativeThemeCocoa.mm

@@ -2690,15 +2690,6 @@ Maybe<nsNativeThemeCocoa::WidgetInfo> nsNativeThemeCocoa::ComputeWidgetInfo(
   EventStates eventState = GetContentState(aFrame, aAppearance);
 
   switch (aAppearance) {
-    case StyleAppearance::Dialog:
-      if (IsWindowSheet(aFrame)) {
-        if (VibrancyManager::SystemSupportsVibrancy()) {
-          return Nothing();
-        }
-        return Some(WidgetInfo::SheetBackground());
-      }
-      return Some(WidgetInfo::DialogBackground());
-
     case StyleAppearance::Menupopup:
       if (VibrancyManager::SystemSupportsVibrancy()) {
         return Nothing();
@@ -3371,12 +3362,6 @@ bool nsNativeThemeCocoa::CreateWebRenderCommandsForWidget(
   //  - If the case in DrawWidgetBackground draws something complicated for the
   //    given widget type, return false here.
   switch (aAppearance) {
-    case StyleAppearance::Dialog:
-      if (IsWindowSheet(aFrame) && VibrancyManager::SystemSupportsVibrancy()) {
-        return true;
-      }
-      return false;
-
     case StyleAppearance::Menupopup:
       if (VibrancyManager::SystemSupportsVibrancy()) {
         return true;
@@ -4366,10 +4351,8 @@ nsITheme::Transparency nsNativeThemeCocoa::GetWidgetTransparency(nsIFrame* aFram
   switch (aAppearance) {
     case StyleAppearance::Menupopup:
     case StyleAppearance::Tooltip:
-      return eTransparent;
-
     case StyleAppearance::Dialog:
-      return IsWindowSheet(aFrame) ? eTransparent : eOpaque;
+      return eTransparent;
 
     case StyleAppearance::ScrollbarSmall:
     case StyleAppearance::Scrollbar: