Make a translucent background color really see-through on macOS

SetBackgroundColor only set NSWindow.backgroundColor. A window that stays opaque composites that color over black, and a content view controller with a backing of its own - a FlutterViewController is opaque black - covers it anyway. A color with alpha now also clears NSWindow.opaque and is handed to a content view controller that has a backgroundColor.

Found with the Flutter floating_toolbar_example: the frameless toolbar window showed a black rectangle around its rounded content.

Author: lijy91

src/platform/macos/window_macos.mm

@@ -730,6 +730,17 @@ static void NativeApiUpdateWindowClass(NSWindow* window, bool non_activating) {
                                       blue:color.b / 255.0
                                      alpha:color.a / 255.0];
   [pimpl_->ns_window_ setBackgroundColor:nsColor];
+  // A translucent background only shows through a window that says it is not
+  // opaque; without this AppKit composites it over black.
+  [pimpl_->ns_window_ setOpaque:color.a == 255];
+
+  // The content may paint a backing of its own over the window's background. A
+  // view controller that has a background colour takes the same one: that is
+  // what makes a FlutterViewController, opaque black by default, see-through.
+  NSViewController* content = [pimpl_->ns_window_ contentViewController];
+  if ([content respondsToSelector:@selector(setBackgroundColor:)]) {
+    [content performSelector:@selector(setBackgroundColor:) withObject:nsColor];
+  }
 }
 
 Color Window::GetBackgroundColor() const {

src/window.h

@@ -847,7 +847,10 @@ class Window : public NativeObjectProvider, public std::enable_shared_from_this<
    *
    * @note Platform behavior may vary:
    * - Windows: Sets the window background brush color
-   * - macOS: Sets the window backgroundColor property
+   * - macOS: Sets the window backgroundColor property. A color with alpha also
+   *   makes the window non-opaque, so that it really is see-through, and is
+   *   handed to a content view controller that paints a backing of its own
+   *   (a Flutter view is opaque black otherwise).
    * - Linux: Sets the window background color via GTK/X11
    */
   void SetBackgroundColor(const Color& color);