Bug 1870512 - Simplify ShouldAttachToTopLevel(), and revert patch for bug 1869891. r=tnikkel

Definitely not for 122, but I think we should try this. Note that this
doesn't affect nsMenuPopupFrames, which create the widget on their own
here:

  https://searchfox.org/mozilla-central/rev/07937a5d00e0f65611d8b3bd2992c7603aeaa70d/layout/xul/nsMenuPopupFrame.cpp#266

It only affects windows with popup=yes, which right now are only the
nsXULAlerts windows.

I'll look into trying to unify the macOS set-up, which right now
unconditionally uses this (presumably to get an nsChildView for each
top-level nsCocoaWindow). Maybe that can be handled internally by the
widget layer, we'll see.

Differential Revision: https://phabricator.services.mozilla.com/D196664

Author: emilio

layout/base/PresShell.cpp

@@ -11607,14 +11607,6 @@ void PresShell::SyncWindowProperties(bool aSync) {
     windowWidget->SetTransparencyMode(mode);
     windowWidget->SetWindowShadowStyle(shadow);
 
-    nsCOMPtr<nsIWidget> viewWidget = view->GetWidget();
-    if (viewWidget != windowWidget) {
-      // Happens on macOS (where we have an nsChildView as a child of an
-      // nsCocoaWindow), and in popup=yes windows in other platforms, in
-      // practice.
-      viewWidget->SetTransparencyMode(mode);
-    }
-
     // For macOS, apply color scheme overrides to the top level window widget.
     if (auto scheme = pc->GetOverriddenOrEmbedderColorScheme()) {
       windowWidget->SetColorScheme(scheme);

layout/base/nsDocumentViewer.cpp

@@ -3252,33 +3252,22 @@ bool nsDocumentViewer::ShouldAttachToTopLevel() {
     return false;
   }
 
-  if (!mContainer) {
-    return false;
-  }
-
   // We always attach when using puppet widgets
   if (nsIWidget::UsePuppetWidgets()) {
     return true;
   }
 
-#if defined(XP_WIN) || defined(MOZ_WIDGET_GTK) || \
-    defined(MOZ_WIDGET_ANDROID) || defined(MOZ_WIDGET_UIKIT)
-  if (!mPresContext) {
-    return false;
-  }
-
-  // On windows, in the parent process we also attach, but just to
-  // chrome items
-  auto winType = mParentWidget->GetWindowType();
-  if ((winType == widget::WindowType::TopLevel ||
-       winType == widget::WindowType::Dialog ||
-       winType == widget::WindowType::Invisible) &&
-      mPresContext->IsChrome()) {
-    return true;
-  }
-#endif
-
+  // FIXME(emilio): Can we unify this between macOS and aother platforms?
+#ifdef XP_MACOSX
   return false;
+#else
+#  ifdef DEBUG
+  nsIWidgetListener* parentListener = mParentWidget->GetWidgetListener();
+  MOZ_ASSERT(!parentListener || !parentListener->GetView(),
+             "Expect a top level widget");
+#  endif
+  return true;
+#endif
 }
 
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