Shadow for blurred/transparent window on macOS?

[alphaArgon]

The shadow was technically disabled against WindowServer bugs. I made a PR #27403 to add it back but here are some points to discuss.

I believe people would happy having the shadow back, but to be more specific, should the shadow always be visible? As mentioned in the PR, applying a shadow to a fully transparent window might look awkward. Compared to other GUIs, macOS window shadows are much heavier.

The Mac Terminal window always has a shadow. Setting its background color to 0% opacity still leaves a slight tint.

My opinion is that all opaque and blurred windows should always have a shadow;transparent windows should have a shadow if the background opacity is >= 25%.