#40 Tahoe 26 Liquid Glass integration: theme picker hooks, layer-background mode, focus-report leak fix

Companion to cmux #2647 (Adopt macOS 26 Liquid Glass design) and cmux #2646 (sidebar sections). Both cmux PRs pin to this ghostty SHA; merging here lets them bump pointer back to manaflow-ai/ghostty#main.

Summary

Three logical groups, combined into one PR because cmux tests them together:

cmux theme picker integration

• Add cmux theme picker helper hooks
• Fix cmux theme picker preview writes
• Improve cmux theme picker footer contrast
• Respect system theme in cmux picker
• Skip theme detection in cmux picker
• Match Ghostty theme picker startup
• Harden cmux theme override writes

macos-background-from-layer config (needed for Liquid Glass translucent backgrounds)

• Add macos-background-from-layer config flag
• Skip fullscreen bg draw call in layer-background mode

Focus-report leak fix (ghostty-org/ghostty #2446)

• Seed initial focus state and avoid startup focus-report leak
• Merge of origin/main into the focus-reporting-leak branch (11th commit)

Test plan

• zig build clean
• cmux theme picker opens, renders cmux-owned theme list, preview writes land without stomping user config
• With macos-background-from-layer = true, Ghostty skips the fullscreen background draw — required for the SwiftUI .glass material in cmux Liquid Glass mode to show through
• Boot Ghostty with mode 2031 enabled; verify no spurious focus-out report at startup (issue #2446)
• cmux PRs #2647 and #2646 green after pointer bumps to merged SHA

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Summary by cubic

Adds macOS layer-provided background support for Tahoe 26 Liquid Glass, integrates cmux theme picker hooks, and fixes a startup focus-report leak. The renderer can now skip its fullscreen background pass on macOS, and surfaces seed initial focus so DECSET 1004 doesn't emit at startup.

• New Features

  • macos-background-from-layer (macOS): lets the host CALayer provide the background; renderer zeros bg alpha and skips the fullscreen fill. Enables Liquid Glass behavior.
  • Theme picker hooks for cmux: picker renders cmux theme data with safe preview writes, startup parity, and system theme respect.

• Bug Fixes

  • Focus-report leak: surfaces accept an initial focused state and seed the terminal before IO. Enabling DECSET 1004 no longer sends an immediate focus sequence.