#21655 cmux-next: terminals show background art through the glass, like the other panes

With background art on, terminal panes painted their theme background solid over it, while chat, pages and the sidebar beside them stayed clear and showed the art through the window's one legible tint (#18358). Terminals now do the same: over art a terminal is glass like every other pane.

What changes

• Surfaces. Art makes a window see-through at any appearance.backgroundOpacity (WindowBackdrop), but Ghostty's background-opacity stayed 1, so GhosttyRuntimeSurfacePolicy left the surfaces opaque. With art shown, either appearance.background or a workspace's own pick (cx-t2x.5), the surfaces now draw a transparent default background, as they already do in a translucent window or under a terminal background override. The flag only changes when art turns on or off, so moving between paintings never reloads Ghostty.
• Host. TerminalHostView read the config's opacity to decide whether to paint. It now reads WindowBackdrop.current, the art-aware check the panes use (panesPaintBackground). An opaque window still paints the theme background.
• A terminal with its own theme. Where the window is see-through, a terminal whose room, workspace or own theme differs from the panes around it paints that theme's background at the window's tint. Before, it showed the window's tint in the window theme's color, which could put a light theme's dark text on a dark tint. Over art the tint is never below legibleTintOpacity, the floor that keeps text readable on any art.
• Readability. Text contrast comes from libghostty's minimum-contrast, which since ghostty-next PR 36 measures against the real background at background-opacity 0. That is the same path a translucent window already takes (GhosttyGlassMinimumContrastTests).

Glass tour

A tour can now say which cmux-next.json the app launches with (settings). tour.py writes it to the tour's private work directory and points CMUX_NEXT_CONFIG_FILE at it, so cmux settings set in a tour writes there and never to the runner's own settings. The new glass tour opens a terminal with plain and ANSI colored text next to an agent chat, over the Great Wave. It then switches through Clay Orange, Soft Blue and their light pairs, over art and opaque. path_map.py picks it for changes to themes, glass, backdrops, window and surface backgrounds, the terminal host and Resources/cmux-themes.

Testing

• swift build --target CmuxNextApp on cmux16 at e7797dee80f: builds, no new warnings in the changed files.
• tour.py --check scripts/cmux-next/media/tours: 4 tours, 31 steps, ok. path_map.py maps this diff to glass and core, and Resources/cmux-themes/* to glass.
• No unit test added (cmux-next no-unit-tests rule). The proof is the glass tour's before and after media on this PR.

Changelog

Fixed: Terminals show background art through the glass like the other panes, instead of painting a solid theme background over it

🤖 Generated with Claude Code

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Summary by cubic

Terminals now show background art through the glass like the other panes, instead of painting a solid theme background over it.

• Each terminal host builds the window backdrop around it, reports its window's see-through state to the runtime, and repaints when the scope around it changes, so workspace or room theme changes and moves stay correct.
• A terminal whose theme differs from the panes around it paints that theme's background at the window's tint, keeping its text readable over art; Reduce Transparency is honored.
• Ghostty config reloads happen only when the first host turns see-through, the last stops, or a host moves between superviews, so moving between paintings never reloads.
• Tours can launch with their own cmux-next.json in a private work directory; a new glass tour covers terminals and cmux themes over art and opaque.