Two root causes for issue #879:
1. GhosttyNSView was missing makeBackingLayer(), so AppKit provided a
generic CALayer as the view's backing layer. libghostty expects
(view.layer as? CAMetalLayer) != nil to set up Metal rendering on
the existing layer. Without a CAMetalLayer backing layer, the Metal
surface defaulted to isOpaque=true, making the terminal area fully
opaque regardless of background-opacity config.
Fix: override makeBackingLayer() to return a CAMetalLayer with
isOpaque=false and bgra8Unorm pixel format (matching standalone
Ghostty's SurfaceView behavior).
2. ghostty_set_window_background_blur(app, window) is exposed in
ghostty.h but was never called in cmux. Without this call the
macOS window never gets the NSVisualEffectView blur backdrop that
background-blur requires.
Fix: add applyWindowBlurIfNeeded() on GhosttyApp that reads
background-blur from config via ghostty_config_get and calls
ghostty_set_window_background_blur when the value is non-zero.
Called from applyBackgroundToKeyWindow() and
applyWindowBackgroundIfActive() whenever the window is made
transparent.
Fixes#879
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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