gpui_macos: Fix window blur on macOS 26+ by using UnderWindowBackground

[neo773]

Summary

WindowBackgroundAppearance::Blurred renders with no blur on macOS 26 and 27.

BlurredView is an NSVisualEffectView set to NSVisualEffectMaterial::Selection, and blurred_view_update_layer walks the resulting layer tree to strip the material's own backgrounds and its colorSaturate filter.

On macOS 26+, Selection no longer vends a CABackdropLayer — so there is nothing to walk, and nothing to blur.

Evidence

Probed on macOS 27.0, reading the live layer tree of the BlurredView in a running GPUI app configured with WindowBackgroundAppearance::Blurred:

layer    = NSViewBackingLayer
filters  = 0
sublayers = 1  → ["CALayer"]

No CABackdropLayer, and no filters for remove_layer_background to find. On macOS 25 and earlier the same view carries a backdrop sublayer plus the Blur and colorSaturate filters that code is written against.

Fix

Use NSVisualEffectMaterial::UnderWindowBackground, which still vends a backdrop on macOS 26+ and is the material AppKit documents for the area behind a window's own background.

Also state NSVisualEffectBlendingMode::BehindWindow explicitly. It is already the default, but sampling the desktop rather than the window's own content is this view's entire purpose, and leaving it implicit makes the view's behaviour depend on a default that has now changed once.

remove_layer_background is untouched and still applies to the new material's layer tree.

Verification

Built and run on macOS 27.0 against a GPUI application using WindowBackgroundAppearance::Blurred — solid/unblurred window before, blur restored after. macOS 25 and earlier are unaffected in the same build.

Release Notes:

• Fixed blurred window backgrounds rendering unblurred on macOS 26 and later.

[dinocosta]

Hey @neo773 ! Thank you for suggesting these changes 🙂

Before we have someone review it, would you be able attach screenshots from a simple GPUI example showing the before and after? As stated in our Contributing Guide, any UI change should have screenshots or screen recordings attached.

Thanks!