windows-canvas optimized rendering and text layout

[Kenny Kerr (kennykerr)]

Adds demand-driven and state-driven controls to windows-reactor, plus a TextLayout API and invalidate / text layout samples. This makes it far simpler to produce efficient text rendering and demand-driven canvas rendering in general.

[Copilot]

Pull request overview

This PR extends the windows-reactor + windows-canvas integration to support demand-driven rendering (size-driven and state-driven invalidation) and adds a higher-level TextLayout API for measured/hit-tested text that can be shaped once and reused.

Changes:

• Add demand-driven canvas and state-driven canvas_invalidated (via Invalidator) to windows-reactor’s canvas bridge.
• Add TextLayout + WordWrapping to windows-canvas, plus DrawingSession::draw_text_layout.
• Update docs, tests, and samples to demonstrate demand-driven rendering + text layout + invalidation patterns.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/crates/windows-reactor.md	Documents new demand-driven rendering APIs and recommended patterns.
docs/crates/windows-canvas.md	Adds docs for demand-driven rendering, TextLayout, wrapping, and sample references.
crates/tools/bindings/src/canvas.txt	Extends bindgen filter to include DirectWrite layout APIs and constants.
crates/tests/libs/canvas/src/lib.rs	Adds Rect::offset tests and new TextLayout behavior tests.
crates/samples/canvas/samples/src/lib.rs	Switches sample harness default to demand-driven canvas; adds run_animated.
crates/samples/canvas/samples/examples/transform.rs	Uses run_animated since it animates each frame.
crates/samples/canvas/samples/examples/text_layout.rs	New example demonstrating TextLayout measurement + demand-driven repaint.
crates/samples/canvas/samples/examples/invalidate.rs	New example demonstrating explicit invalidation-driven repaint.
crates/samples/canvas/hit_test/src/main.rs	Converts hit-test sample to invalidate-on-pointer events instead of per-frame redraw.
crates/samples/canvas/editor/src/main.rs	Converts editor sample to invalidate-on-state-change rendering.
crates/libs/reactor/src/lib.rs	Re-exports canvas, canvas_invalidated, and Invalidator under the canvas feature.
crates/libs/reactor/src/canvas_bridge.rs	Implements demand-vs-continuous render modes and the Invalidator hook/API.
crates/libs/canvas/src/types.rs	Adds Rect::offset.
crates/libs/canvas/src/text.rs	Adds WordWrapping, TextLayout, metrics/hit-test APIs.
crates/libs/canvas/src/session.rs	Adds DrawingSession::draw_text_layout.
crates/libs/canvas/src/bindings.rs	Generated bindings updated for DirectWrite layout + metrics/hit-test structs/methods.

[Copilot]

Pull request overview

Copilot reviewed 40 out of 40 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (6)

crates/libs/canvas/src/text.rs:241

• hit_test_point discards the HRESULT from IDWriteTextLayout::HitTestPoint. On failure (e.g., invalid input), the returned HitTest will be based on default/undefined metrics. Returning Result<HitTest> (or validating inputs / asserting success) would prevent silent incorrect results.

        unsafe {
            _ = self
                .raw
                .HitTestPoint(point.x, point.y, &mut trailing, &mut inside, &mut m);
        }

crates/libs/canvas/src/text.rs:262

• caret_bounds discards the HRESULT from IDWriteTextLayout::HitTestTextPosition. This can fail for out-of-range UTF-16 positions; returning a Rect anyway can silently produce incorrect caret geometry. Consider returning Result<Rect> (or explicitly clamping/validating position).

        unsafe {
            _ = self
                .raw
                .HitTestTextPosition(position, trailing, &mut x, &mut y, &mut m);
        }

crates/libs/reactor/src/canvas_bridge.rs:401

• CanvasImageSource::draw now takes a callback returning Result<()>, but the doc example above still uses a closure returning () and calls unwrap() inside it. Please update the example to return Ok(()) and use ? for fallible brush creation so the docs match the API contract.

    pub fn draw(
        &self,
        clear: ColorF,
        f: impl FnOnce(&DrawingSession<'_>) -> Result<()>,
    ) -> Result<bool> {

crates/libs/reactor/src/canvas_bridge.rs:92

• The example for animated_canvas no longer matches the function signature: the draw closure now returns Result<()>, so the snippet should return Ok(()) (and ideally use ? for fallible resource creation).

This issue also appears on line 397 of the same file.

/// animated_canvas(|ctx| {
///     ctx.clear(ColorF::CORNFLOWER_BLUE);
///     ctx.fill_ellipse(&ellipse, &brush);
/// })
/// ```

crates/libs/canvas/src/text.rs:219

• TextLayout::metrics ignores the HRESULT from IDWriteTextLayout::GetMetrics. If the call fails, this returns default/garbage metrics with no signal to the caller. Since this is a new public API, consider making it fallible (e.g. metrics(&self) -> Result<TextMetrics>) or at least asserting/handling failure explicitly.

This issue also appears in the following locations of the same file:

• line 237
• line 258

    pub fn metrics(&self) -> TextMetrics {
        let mut m = DWRITE_TEXT_METRICS::default();
        unsafe { _ = self.raw.GetMetrics(&mut m) };
        TextMetrics {

crates/samples/canvas/text_layout/src/main.rs:49

• The on-screen readout reports the TextLayout metrics as "px", but the TextMetrics values are in device-independent pixels (DIPs). The unit label should match the actual units to avoid confusion.