build: fizzy builds on dvui's SDL3 backend again, and CI keeps it

[foxnne]

What changes

zig build -Dnative-backend=sdl3 (fizzy on dvui's own SDL3 backend) compiles and runs again, and CI now builds it on macOS so it can't break unnoticed again.

On main it stopped compiling. The backend package's platform module reached into things only fizzy's own backend (backend/src/SDLBackend.zig) has:

• win.backend.impl.viewports, in platform/window.zig: restyling and reskinning each float's own window.
• win.backend.impl.window_skin, in platform/window.zig.
• back.present_hook, in platform/macos_monitor.zig.
• fizzy_native_transaction_begin / fizzy_native_viewport_transact, in platform/macos_monitor.zig. These are defined in the backend's macos_monitor.m, so the mode failed at link time once the first three compiled.

All of these now sit behind one comptime flag, platform.own_backend, which is true only when @import("backend") is the package's own backend: that is, the one that declares platform. On dvui's backend those parts do nothing. SDLBackend asserts the flag at comptime, so fizzy's own build fails rather than silently losing them.

Why one flag instead of a @hasField per field: #324 renames viewports to viewport_slots. With a per-field @hasField("viewports"), that rename would turn the guard false on fizzy's own backend, and floats would quietly stop being reskinned. With the flag, a renamed field still breaks fizzy's build.

The transaction calls aren't just a link fix. macosAppPreBeginSync opens a Core Animation transaction that only the present hook closes. Without the guard, dvui's backend would have left that transaction open through a full-screen transition.

Full screen on dvui's backend: AppKit's own, with fizzy's chrome

Full screen was broken on this mode. The macOS monitor (platform/macos_monitor.zig + macos/window_monitor.m) takes full screen and zooms over from AppKit. It animates the window itself and draws each step from the backend's begin and present hooks inside a Core Animation transaction, which only fizzy's backend has. On dvui's backend those steps were drawn out of time with the window.

macos_monitor.install now leaves that out on dvui's backend. The window goes in and out as AppKit and SDL take it, and every query (in_fullscreen_space, zoomed, titlebar inset) answers from AppKit's own state.

One piece of the monitor is about fizzy's chrome, not its backend: the setStyleMask: guard. SDL puts its own style on the window at will-enter and did-exit, and styleTitled puts fizzy's back. Without the guard, AppKit keeps the content size across each change. Measured with the monitor left out entirely, the window came back from each round trip 40 pt shorter: 800 → 760 → 720. fizzy_macos_window_keep_full_size_content installs that guard alone, and on dvui's backend it is all install does.

What stays on dvui's backend: the frosted or Liquid Glass window, the transparent title-less titlebar and the titlebar hit test. They are plain AppKit calls that work on any SDL window. What goes: floats as OS windows, the window skin on float windows, custom GPU programs, fizzy's own full-screen and zoom animation, live-resize sync and the health counters.

Option considered: drop the mode

The other way to fix this was to remove -Dnative-backend=sdl3 entirely: NativeBackend, the build option, and the supported/@hasDecl fallbacks. Fizzy's own app depends on its backend's viewports, glass and window skin, and nothing shipped on the sdl3 mode. I built that version and it passed every gate.

The decision went to keeping the mode. A dvui app that uses fizzy's backend package on the stock dvui backend is in the same position as fizzy in this mode (example-app's coming run-dvui). Building fizzy this way is the cheapest proof that the platform pieces still work for such an app. It costs one guard per backend-only reach, plus one CI step.

CI

The macOS leg of the test matrix gets two steps: fetch with -Dnative-backend=sdl3, then zig build -Dnative-backend=sdl3 -Dno-emit. It runs on macOS only because all four breakages were in macOS-only branches, which a Linux build never analyses. Cost: dvui's own SDL is compiled on that leg. It isn't in the leg's saved Zig cache until the next build.zig.zon change re-keys it.

Coordination

#324 landed first, carrying the stand-in viewports for dvui's backend (viewports_none). Main is merged into this branch (1dca6885): the two own_backend-guarded loops in platform/window.zig now read impl.viewport_slots, and zig build, -Dnative-backend=sdl3 and the gates pass on the merge.

SDK impact

• None. The backend package and platform aren't part of the plugin boundary, and test-sdk-version passes.

Verified

• macOS (arm64, macOS 26, notched display):

  • Gates pass: zig build, zig build test (514/515, 1 skipped), zig build test-integration (44/44 steps, 358/358), zig build check-web, zig build test-sdk-version.

  • zig build -Dnative-backend=sdl3 builds, links and runs.

  • Full screen, driven through the agent plugin (fizzyedit/agent's fizzy-mcp, in an isolated --profile). fizzy.toggleFullScreen ran four times; after each toggle, fizzy's layout (fizzy_screen) and AppKit's window frames (CGWindowList) were sampled for 3.5 s. The start frame is 300,204 1200×800 in every run:

    build	after each exit	full screen
    sdl3, before this PR's full-screen change (monitor installed)	back to 1200×800, fizzy's own animation drawn out of time	0,39 1800×1130
    sdl3, monitor left out entirely	800 → 760 → 720, moving down 40 pt each time	0,39 1800×1130
    sdl3, this PR	back to 300,204 1200×800 every time, AppKit's animation	0,39 1800×1130
    fizzy's own backend, this PR	back to 300,204 1200×800 every time, its own animation (layout steps through 3140 → 3600 px)	0,39 1800×1130

    The 39 pt at the top in full screen is the area below the notch, the same on both backends. Fizzy's layout follows each way: full width with the titlebar strip collapsed in full screen, the strip back after.

  • Screenshots after a round trip on the sdl3 build show the frosted window with its transparent titlebar and traffic lights.

  • The own_backend assert works: pointing the flag at a declaration that doesn't exist fails fizzy's own build.

• Windows: zig build -Dtarget=x86_64-windows-gnu -Dno-emit cross-compiles. It compiles; whether it runs wasn't tested.
• Linux: CI. Only linux_titlebar.zig is relevant there, and it is unchanged.
• Web: check-web.

Follow-ups

• Fizzy on dvui's backend on Windows and Linux isn't built by CI. The platform's reach into the backend there is only impl.window, which dvui's backend also has.
• FIZZY_VERDICT writes nothing on dvui's backend: verdicts read fizzy's backend's health counters, which dvui's backend doesn't have. The soak tape itself plays there (found by the "Floating glass windows" session).
• A window-only capture of the sdl3 build in full screen shows the titlebar strip with the traffic lights at the top. AppKit normally reveals that strip on hover, and a window capture may draw it regardless; not checked on screen.
• Zoom (double-clicking the titlebar) on dvui's backend is also AppKit's own now, with the same guard. It hasn't been measured.

🤖 Generated with Claude Code