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Back as a navigation request: App::on_back, BackAtRoot, NavStack and docs/navigation.md; renew the text-input enable on a press; fit an oversized window_size_hint to the screen

Three changes that came out of making the shell's back button work across Eydos the way it does on Android. The first is the feature; the other two are gaps it exposed.
Back is no longer Escape. forge turns its back gesture and button into the `XF86Back` key for the focused client, and ltk treated that key as an alias of `Escape`: close transient UI, else drop the focus and reach `on_key_with_modifiers`. That is "cancel", and it is all an app could get: nothing navigated, and nothing ever left the app. `XF86Back` is now a back request with its own order, one layer per request, top-most first: transient UI the runtime owns (open xdg-popup overlays, the built-in context menu, a text selection, the top-most pressable carrying an `on_escape`, i.e. a dialog's cancel), then text-input focus, then the new `App::on_back`, then `on_key_with_modifiers( XF86_Back )` so an app that bound the key itself keeps receiving it (crustace does, for its launcher), then `App::back_at_root`. The first step is what Escape already did; it moves out of `handle_key_escape` into `AppData::dismiss_transient` in the new `input/keyboard/back.rs`, which both paths call, so Escape behaves exactly as before. Text focus comes before the app on purpose: with a keyboard covering half the screen, back means "put the keyboard away", and navigating instead would discard what the user was typing. A compositor that folds its keyboard itself, as forge now does, only lets the next request through, which makes this step the fallback for the ones that do not.
`App::on_back` returns a `BackResponse`: `Message( msg )` with the message that undoes the top-most piece of navigation state, `Unhandled` at the root, or `Consumed` for a host whose back stack lives elsewhere and cannot be inspected ahead of time. `Consumed` exists for rustdroid: an Android app keeps its activities, fragments and back callbacks in its own code, so the host can only hand the request over. `App::back_at_root` then picks `Minimize` (the default), `Close` (through `on_close_requested`, which can still veto) or `Ignore`. Minimising is what a root activity does on Android since version 12 and what suits an app the user comes back to: a browser keeps its tabs, a call goes on. It is also the single point where the two stacks meet: the navigation stack belongs to the app's model, the task stack to the compositor, and `xdg_toplevel.set_minimized` is the one standard request between them, so no custom protocol carries navigation state out of the process and clients that are not ltk apps keep working. Only `ShellMode::Window` surfaces react at the root; a panel or a lock screen that minimised itself would be a bug.
Three consequences of back being a navigation request rather than input: the key leaves the default `App::key_repeats` set, since a held button would empty the whole stack; it is no longer reported through `App::on_raw_key`, so a host forwarding raw keys into an embedded web view or guest toolkit stops leaking it as a keystroke; and the mouse's back button (`BTN_SIDE` / `BTN_BACK`) enters through the same `handle_back`. `focused_text_input` becomes `pub( crate )` for the new module.
The answer is derived, not recorded, and that is the design decision worth writing down. The runtime keeps no navigation stack of its own. Its entries would have to be messages, which are not serialisable (the trait puts no serde bound on `Message`, by design), so it could not survive a relaunch; and a list of "things to undo" drifts the first time the user leaves a screen some other way, through its own close button or a link, leaving a back press that does nothing visible. An answer computed from the model cannot disagree with it. This is also what ties back to the session persistence added earlier: navigation state is both what `on_back` reads and what `save_state` should persist, and keeping the two the same is what makes back retrace the same places after a session restore or a crash, with nothing extra to serialise. Persist less and a restored app opens on the right screen with nothing behind it; persist more and it restores a menu the user had closed.
`NavStack<R>` packages that rule for the common case, an app made of screens. It is a plain value the app keeps in its model, never seen by the runtime: `push`, `pop`, `replace` (the sideways move back should not retrace), `pop_to_root`, `reset`, `current`, `current_mut`, `iter`, `depth`, `can_pop`, and `back( msg )`, which answers `on_back` directly. It is stored as a root plus the routes above it rather than as a `Vec`, so "never empty" holds by construction, including after deserialising an old or damaged state file, and it derives serde whenever `R` does. Six unit tests cover it, a serde round trip among them, and its rustdoc example is a complete `App` where one object answers `on_back`, `save_state` and `restore_state`.
Documentation, since the point is for app authors to take navigation state into account. The new `docs/navigation.md` is the reference: transient UI versus navigation state and the test that separates them, the back order and the reason for each step, why the answer is derived, how it lines up with session state, `NavStack`, apps that embed a web view (whose page history is navigation state too, to be persisted with ltk-webkit's `session_state`), hosts with a foreign back stack, the root, who owns which stack, and an author's checklist. `architecture.md` gains a group in the trait surface and a paragraph in the state patterns, `onboarding.md` and the README index link to it, and in the cookbook the multi-screen recipe now uses `NavStack` with `on_back` and persists the stack without serde, which keeps it compiled by the Markdown doctest harness (that harness links `ltk` alone, so the serde example in `navigation.md` is marked `ignore` and compiled as `NavStack`'s own doctest instead). `BackResponse`, `BackAtRoot` and `NavStack` are exported at the crate root and in the `ltk::window` facade.
Behaviour change for existing apps, also recorded under Changed: a window app that implements neither `on_back` nor the key is now minimised on back at its root, where nothing happened before; `BackAtRoot::Ignore` restores that. With a text input focused, back only drops the focus and the app sees the next request, where it used to get the key in the same press.
Renew the text-input enable on a press. Moving the focus between two text fields, or pressing the one already focused, only committed the refreshed content type on the existing `zwp_text_input_v3`. `activate_text_input` now sends `enable` first, as GTK does when a focused entry is clicked. A compositor that folded its on-screen keyboard while the field kept the focus, which is what back now does, has no other way to learn that the user wants it again: the text-input never stopped being active, so there was nothing for it to observe. ltk sends neither surrounding text nor a cursor rectangle, so the only state `enable` resets is the content type set right after it.
Fit an oversized `window_size_hint` to the screen. The hint is pinned as `min_size == max_size` before the first commit, when no output is known yet, so an app asking for a phone-shaped 720x1440 window on a 1080p desktop got a toplevel taller than the screen; rustdroid carried a dead branch that tried to avoid this from a screen size nothing could give it. The first configure is the earliest point the hint can be checked, and the protocol has the right input for it: `xdg_toplevel.configure_bounds`, exposed by sctk as `WindowConfigure::suggested_bounds` and ignored until now. The new `event_loop/window_fit.rs` holds `fit_within`, which scales a size down keeping its aspect ratio until it fits and leaves one that fits untouched (four unit tests), and `AppData::fit_size_hint`, run once behind the new `pending_size_hint_fit` latch: it uses the compositor's bounds when sent and otherwise 90 % of the first output's logical size, since panels and decorations are unknown from the client side. When the hint has to shrink, `min_size` (and `max_size` for a non-resizable window) is re-pinned to the fitted size, which is kept in `fitted_size_hint` and replaces the app's hint as the fallback for later configures that carry no size; without that the window would grow back on the next one. On that one configure the fitted size wins over the compositor's `new_size`, which was derived from the oversized pin. `App::window_size_hint` documents the rule with the worked example (720x1440 opens as 486x972 on a 1080p output).
The back order needs a live Wayland connection to exercise and has no test; the unit tests cover `NavStack` and `fit_within`.

2026-09-20 23:23:26 +02:00

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ltk

ltk is a public Rust UI toolkit for Wayland applications.

It is developed by Liberux as part of the Eydos stack, where it powers shell and application surfaces, but it is published as a reusable library for third-party developers building their own Wayland software.

Being written in Rust is also part of the project's value proposition:

memory safety without a garbage collector
predictable resource lifetimes through ownership and borrowing
good control over allocations and data movement in rendering-heavy code
a strong fit for low-level UI, graphics, and system-integration work

For a Wayland toolkit, that combination is useful in practice: it reduces an entire class of memory-management bugs common in lower-level UI stacks while still allowing tight control over performance-sensitive paths.

What It Is

ltk is a lightweight, declarative toolkit with an Elm-shaped model:

implement App
return an Element<Msg> tree from view()
update your state in update()
run the event loop with ltk::run(app)

The runtime handles layout, drawing, input dispatch, focus, overlays, and backend selection between GLES and software rendering.

What It Is Not

ltk is not:

a browser UI toolkit
a cross-platform desktop toolkit
a general-purpose web-style framework

Today it is specifically a Wayland toolkit. If you are building native Wayland applications, panels, launchers, lock screens, or other shell-adjacent surfaces, it is in scope. If you need Windows, macOS, or browser targets, it is not.

Project Status

ltk is a public library intended for third-party use, but it is still shaped by real production needs inside the Liberux / Eydos ecosystem.

That means:

the API is usable for external applications today
the project is optimized first for native Wayland workloads
some advanced APIs are still more shell-oriented than app-oriented
public documentation and examples are present, but the project is not trying to present itself as a cross-platform beginner toolkit

If you are evaluating ltk for a third-party application, the right mental model is "public Wayland toolkit with production consumers" rather than "experimental demo crate".

Why Third Parties Might Use It

ltk is designed around a few practical goals:

low idle wakeups and event-driven redraws
partial redraws and damage tracking
a simple declarative tree instead of retained widgets
direct support for normal windows, layer-shell, and ext-session-lock surfaces
a runtime-free core (ltk::core::UiSurface) for embedding layout and drawing without ltk::run()

This makes it especially relevant for:

Wayland applications
mobile-first Linux shells
launchers and dashboards
greeters and lock screens
compositor-side or embedded UI surfaces

Quick Start

Add ltk to your Cargo.toml:

[dependencies]
ltk = { path = "../ltk" }

If you consume ltk from outside this workspace, replace the path dependency with a versioned one.

Minimal app:

use ltk::{ App, Element, button, column, spacer, text };

#[derive(Clone)]
enum Msg
{
    Increment,
}

struct CounterApp
{
    value: u32,
}

impl App for CounterApp
{
    type Message = Msg;

    fn app_id( &self ) -> &str { "net.example.Counter" }
    fn save_state( &self ) -> Option<Vec<u8>> { None }
    fn restore_state( &mut self, _state: Vec<u8> ) {}

    fn view( &self ) -> Element<Msg>
    {
        column::<Msg>()
            .padding( 32.0 )
            .spacing( 16.0 )
            .center_y( true )
            .push( text( "Hello from ltk" ).size( 28.0 ) )
            .push( text( format!( "Count: {}", self.value ) ).size( 18.0 ) )
            .push( spacer() )
            .push( button( "Increment" ).on_press( Msg::Increment ) )
            .into()
    }

    fn update( &mut self, msg: Msg )
    {
        match msg
        {
            Msg::Increment => self.value += 1,
        }
    }
}

fn main()
{
    ltk::run( CounterApp { value: 0 } );
}

app_id names the window and the $XDG_STATE_HOME/<app_id>/ session directory; save_state / restore_state are the opaque bytes ltk persists for session restore and crash recovery — return None when you have nothing to keep.

Requirements

ltk currently assumes:

Rust 1.85 or newer (the toolchain shipped with Debian stable; declared as rust-version in Cargo.toml).
A running Wayland session — there is no X11 backend.
System headers for libwayland, libegl and libxkbcommon at compile time. On Debian / Ubuntu:
```
sudo apt-get install libwayland-dev libegl-dev libxkbcommon-dev pkg-config
```
A usable system font (fonts-sora, fonts-liberation, fonts-dejavu, …). If none is installed ltk falls back to an embedded Sora Regular build with a stderr warning.
A theme named default, installed system-wide (the ltk-theme-default Debian package drops it under /usr/share/ltk/themes/default/) or exposed through LTK_THEMES_DIR for development.

Rendering backend selection is automatic:

GLES when EGL is available (every modern Wayland compositor).
Software fallback otherwise.
Set LTK_FORCE_SOFTWARE=1 to force the software path even when EGL is available — useful for headless test runs and for diagnosing driver-specific bugs.

For development inside this repository:

export LTK_THEMES_DIR="$PWD/themes"
cargo run --example showcase

Examples

Useful entry points in this repository:

cargo run --example showcase
cargo run --example responsive
cargo run --example widgets
cargo run --example inputs
cargo run --example scroll
cargo run --example combo
cargo run --example dialog
cargo run --example sliders
cargo run --example pickers
cargo run --example carousel
cargo run --example clip_path
cargo run --example mini_shell

In general:

start with showcase for a regular app window
use responsive to see the fluid vs physical modes on stock widgets
use widgets to see the core controls
use mini_shell if you need overlays, theme switching, or shell-style composition

Public API Overview

Most applications should start with this subset:

App
Element<Msg>
widgets such as button, text, text_edit, img_widget
layouts such as column, row, stack, grid, spacer
Color
run

More advanced APIs are available when needed:

overlays()
shell_mode() and layer-shell controls
set_channel_sender() and poll_external()
gesture hooks such as on_swipe_*
core::UiSurface
runtime theme APIs

Responsive Design

ltk offers two first-class ways to make an interface adapt to the display, chosen per value or per process:

Fluid — sizes are a fraction of the surface, tracking the short side (width in portrait, height in landscape). Best for full-screen system surfaces. Written with Length::vmin / orient / fluid and bounded with .clamp.
Physical — sizes stay a constant real-world size across displays (the mainstream HiDPI dp model). Best for conventional windowed apps. Written with Length::dp plus set_density.

Stock widgets follow the process-wide mode (set_widget_scaling, fluid by default); an explicit Length on a widget always overrides it. The full mechanics live in docs/architecture.md, a walkthrough in docs/onboarding.md, and the per-item reference in the Length / WidgetScaling rustdoc.

Windows and Shell Surfaces

By default, ltk creates a regular xdg-shell window.

That is the right starting point for:

normal applications
internal tools
prototypes

Switch to layer-shell only when you are building shell surfaces such as:

top bars
docks
homescreens
notifications
greeters
lock screens

For a screen locker, use ShellMode::SessionLock instead of layer-shell: it presents an ext-session-lock-v1 surface that the compositor keeps on top of everything until the app returns true from requested_exit(), which makes the runtime call unlock and lift the lock.

Performance Notes

ltk is designed to sleep when idle and redraw only on real work.

The main rules for downstream applications are:

keep view() pure and cheap
do not perform I/O inside view()
use poll_interval() sparingly
return true from is_animating() only while something is actually moving — note that on the software backend animation redraws are capped at ~30 Hz by default (override with App::cap_software_animation)
cache decoded images and expensive derived state in your app

Set LTK_PERF_WARN=1 during development for opt-in diagnostics: it warns about stuck animations, sustained software-render animation, and low poll_interval values.

The library already provides:

event-driven redraw scheduling
per-surface invalidation
partial redraws for interaction-only changes
GPU and software backends behind the same widget API

Backend Differences

The public API is the same across backends, but visual parity is not perfect yet. The widget tree, layout, hit-testing, text, images, fills, strokes and single-rect / path clipping all paint identically on both paths. The gaps are in gradients, the shadow / backdrop pipeline, and multi-rect clipping: the GLES backend installs the bounding-box union of the rects as its glScissor — a coarse clip that does not cull pixels between disjoint rects — while the software backend clips each rect exactly.

Effects that currently render only on the GLES backend, and degrade on the Software backend:

Gradients (linear and radial, via Canvas::fill_paint_rect) — rendered with dedicated shaders on GLES; on software they collapse to a flat fill from the first stop (tiny-skia can render gradients natively, but that is not wired up yet).
Outer drop shadows (Canvas::fill_shadow_outer) — themed surfaces that declare a Shadow slot show the soft halo on GLES and a flat fill on software.
Inner / inset shadows (Canvas::fill_shadow_inset) — InsetShadow slots paint nothing on software.
Inset shadow blend modes — PlusLighter, Multiply, Screen and Overlay are GLES-only; the GLES Overlay path snapshots the framebuffer and computes the CSS Overlay formula in-shader, which has no software equivalent today.

Calls to these APIs are safe on both backends — they simply produce a flatter appearance under software. No widget panics, returns an error, or skips unrelated drawing.

If your application leans heavily on shadows or inset effects, validate both rendering paths before shipping. Force the software path with:

LTK_FORCE_SOFTWARE=1 cargo run --example showcase

Closing this gap (porting the shadow / inset-shadow pipeline to tiny-skia) is on the roadmap.

Documentation

File	When to read it
`docs/onboarding.md`	First hour with the library — environment, first app, what to ignore at first.
`docs/architecture.md`	Runtime model, overlays, animation, theming, performance and where the cost of a frame lives.
`docs/widgets.md`	Per-widget catalogue: what each one is, when to use it, minimal example, see-also.
`docs/theming.md`	JSON theme schema, slot conventions, runtime APIs.
`docs/backends.md`	Software/GLES capability matrix — what renders identically, what degrades, what is GPU-only.
`docs/navigation.md`	Navigation state and the back button: what back closes and in which order, why the answer is derived from the model, how it lines up with session state, `NavStack`.
`docs/cookbook.md`	Concrete recipes — slide-in panels, password fields, runtime theme toggle, channel-driven state, embedding without `ltk::run`.
`cargo doc --open`	Per-item rustdoc for the public API.
`CHANGELOG.md`	What changed in each release, and what is pending unreleased.
`code_style_guide.md`	The Modified Allman style rules the codebase follows.
`SECURITY.md`	How to report a vulnerability and what is in / out of scope.
`CONTRIBUTING.md`	Build, test, code style, patch shape.

Recommended reading order for a new contributor:

run examples/showcase.rs
read docs/onboarding.md
browse docs/widgets.md for the catalogue
dip into docs/cookbook.md when you hit a specific shape
open docs/architecture.md once you need overlays, animations, or runtime theming.

Relationship to Liberux and Eydos

Liberux is the promoter and primary maintainer of ltk.

The project exists because Eydos needs a native Wayland toolkit for its own shell and application stack, but ltk is intentionally published as a public library rather than kept as a private internal component. Third-party developers are part of the intended audience.

That origin matters because it explains the current priorities:

strong Wayland focus
support for layer-shell and shell-style overlays
attention to mobile power usage
theming and runtime surfaces that fit an operating system environment

License

This project is licensed under LGPL-2.1-only.

That means third parties can use ltk in their own applications, including proprietary ones, subject to the obligations of the GNU Lesser General Public License v2.1. If you are planning a commercial or closed-source product, read the license text carefully and make sure your distribution model complies with it.

See LICENSE.

Third-party assets

ltk's default theme bundles two third-party asset sets that travel under their own licences. Anyone redistributing the toolkit (or a binary that embeds the default theme) must propagate the attributions below.

Symbolic icons under themes/default/icons/catalogue/ come from Streamline's Core Line Free set, distributed under Creative Commons Attribution 4.0 International (CC BY 4.0). © Streamline. Some files have been modified for the symbolic-tinting pipeline; details in themes/default/icons/catalogue/LICENSE.md. Upstream: https://www.streamlinehq.com/icons/core-line-free.
Sora Regular (src/theme/fallback/Sora-Regular.otf) is the embedded font fallback, distributed under SIL Open Font Licence 1.1. © 2019-2020 The Sora Project Authors, Jonathan Barnbrook, Julián Moncada. Upstream: https://github.com/sora-xor/sora-font.

The remaining artwork in the default theme — wallpapers, lockscreens, launcher logo, brand-mark variants and per-application icons — is original to Liberux Labs and travels under the toolkit's own LGPL-2.1-only licence.

The full Debian-style declaration of every asset and its licence lives in debian/copyright; that is the file the .deb ships under /usr/share/doc/libltk*/copyright.

Contributing

Patches and bug reports are welcome. Read CONTRIBUTING.md for the practical mechanics: build prerequisites, how to run tests, the project's Modified Allman code style, and what shape a pull request should take.

For security-sensitive issues see SECURITY.md — please do not file those through the public issue tracker.

If you are evaluating ltk for a third-party product and are unsure whether your use case is in scope, open a discussion before writing code. That is especially useful when you are:

missing an app-facing example,
blocked by a shell-oriented assumption in the API,
trying to understand whether a given platform target is realistic.

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ltk