Add headless UI tests with kittest

README.md

@@ -97,8 +97,39 @@ cargo build --release          # target/release/jsoncanvas-desktop[.exe]
   needed; install `xdg-desktop-portal` and a backend for your desktop if it is
   not already present.
 
-Run the tests with `cargo test` (the model, geometry, Markdown and editing logic
-are covered headless — no display required).
+## Tests
+
+`cargo test` runs everything, headless and without a display. Each module's
+tests are a child module of it in a file of its own — `src/view/tests.rs` for
+`src/view.rs` — so they still reach private state without the module itself
+carrying a thousand lines of test code. The GUI is tested the same way as the
+rest:
+
+* **Widgets** are driven through
+  [`egui_kittest`](https://crates.io/crates/egui_kittest), which builds the real
+  `App` on a real `egui::Context` and finds widgets by the label on them:
+
+  ```rust
+  harness.get_by_label("Close & Fit").click();
+  harness.run();
+  assert_eq!(harness.state().view.editing(), None);
+  ```
+
+* **Canvas gestures** — clicks, drags and the wheel on cards, which are painted
+  rather than built from widgets — go through the `pass` helper in `view.rs`,
+  which runs one frame of `CanvasView::show` against synthetic input at chosen
+  coordinates.
+
+* **How it looks** is covered by snapshot tests, which render a frame with
+  `wgpu` and compare it with the images in `tests/snapshots`. A failure writes
+  `<name>.diff.png` and `<name>.new.png` beside the expected image; after an
+  intentional change, re-record with `UPDATE_SNAPSHOTS=1 cargo test`. Rendering
+  falls back to Mesa's software Vulkan driver, so no GPU is needed.
+
+Driving the built binary with synthetic input (`xdotool` and friends) is *not*
+the way to test this: under a Wayland compositor the X pointer warp is scaled,
+`--sync` can block until it is killed, and window captures come back stale, so
+the results are slow and misleading.
 
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