slint

Declarative GUI toolkit for Rust — SPEC.md's handoff spec names it explicitly ("Rust + Slint + libmpv"), so no alternative was considered. UI layout/styling live in .slint markup (crates/app/ui/app-window.slint), compiled at build time via slint-build (build.rs) into generated Rust types main.rs instantiates with slint::include_modules!().

Pitfalls

• .slint files aren't Rust — cargo fmt/clippy don't touch them; no linting is wired into scripts/check.sh.
• Slint's winit backend allows only one platform/event-loop init per process, bound to its creating thread — a second AppWindow::new() on another thread (e.g. a second #[test]) fails ("platform was initialized in another thread"). All assertions needing a real AppWindow must live in one test function.
• AppWindow::new() needs a working windowing backend + X11/Wayland connection to construct at all (winit initializes its event loop immediately, even without ever showing the window) — it's not display-free. CI runs scripts/test.sh under xvfb-run for this reason (.github/workflows/ci.yml). Property-wiring tests never call .show(), so no compositor/renderer work happens beyond that; actual pixels-on-screen checks stay manual (cargo run -p trango).
• Custom fonts ("Inter", "JetBrains Mono") only render if installed as system fonts; Slint falls back silently otherwise — no fonts are bundled.
• HorizontalLayout/VerticalLayout default to cross-axis-alignment: stretch — children fill the full cross-axis extent unless a layout sets cross-axis-alignment: center (as the top bar does).
• No dashed-border support — dashed empty-state rows are approximated with a solid muted border instead.
• DropArea/DragArea don't relay OS file drops — only in-app DragArea sources fire dropped (confirmed by grepping i-slint-backend-winit 1.17.1 for WindowEvent::DroppedFile handling: there is none), and DataTransfer has no file/path payload type yet (tracking issue). This is why subtitle/translation linking uses an in-app file picker instead of drag-and-drop — see Design decisions.