# Architecture: LVGL

User interfaces should scale well for everything between very large (e.g. 1280x720) and small (e.g. 135x240) displays. Vertical and horizontal layouts are supported.

Two kernel modules cover LVGL:

- **`lvgl-module`** (`Modules/lvgl-module/`, `<lvgl/*.h>`) owns LVGL's lifecycle: init/deinit, the LVGL task loop, and `lvgl_lock()`/`lvgl_try_lock()`/`lvgl_unlock()` mutex-based locking that any task must hold before touching LVGL objects. It bridges Tactility's device model to LVGL indevs (`lvgl/devices/*.h`: `display`, `pointer`, `keyboard`, `trackball`), and provides shared fonts (`lvgl/fonts.h`: Montserrat text sizes, Material Symbols icon sets for statusbar/launcher/shared use) and a few shared widgets (`lvgl/widgets/*.h`: `toolbar`, `spinner`, `sliderbox`).
- **`lvgl-window-manager-module`** (`Modules/lvgl-window-manager-module/`, `<lvgl_window_manager/*.h>`) manages a single stacked window per app instance on top of `lvgl-module`. `window_manager_start()`/`window_manager_stop()` create/tear down the root widget (plus optional chrome from a configured `WindowManagerScreenInitFn`); `window_manager_create()`/`window_manager_remove()` push/pop an app's window and (re)populate it via a `WindowCreateWidgetsFn`. Only the topmost window ever has live widgets - burying and resurfacing a window deletes and rebuilds its widget tree rather than hiding/showing it. That rebuild-on-remove path can run `create_widgets` on a *different* app's thread (whichever app's `window_manager_remove()` call caused this window to resurface), so `create_widgets` must only rebuild already-committed state, never decide what happens next - state transitions belong in the app's own `main()` event loop, driven by real `APP_EVENT_RESULT`s.