macOS ignores window Opacity, alpha frost, and Mask transparency

On macOS, the window transparency features do not work:

• Setting Window.Opacity below 1.0 does not fade the window.
• Setting a semi-transparent Window.BackgroundColour (alpha below 255) does not show the desktop behind the window.
• Assigning Window.Mask does not shape the window; a masked window stays a plain rectangle.

A window can also end up in a partially applied state, for example half transparent with opaque white patches, after controls on the window fire events.

Expected behaviour: fading, see-through, and masked windows, as documented and as demonstrated by the bundled Window Transparency example. Windows 11 renders all three correctly; only macOS is affected. The bundled example and any user project using these features on macOS in Studio 26.10.2 show the broken behaviour.