Draw a cursor, handle the lid, and everything a real config needs

Groundwork for actually daily-driving this: porting the user's Hyprland
config exposed what srdwm couldn't yet express, and testing on a bare TTY
exposed something worse.

A visible mouse cursor. Nothing drew a pointer at all - on a bare TTY the
mouse was simply invisible. It hid because the nested backend runs inside
another compositor, which draws a cursor over srdwm's window; only the DRM
backend, i.e. the actual session path, was affected. A built-in arrow is now
composited above everything on the output the pointer is on. It's a
reviewable ASCII bitmap rather than an XCursor theme: a cursor that is always
present beats a prettier one that sometimes isn't, the same reasoning as
decoration.rs's font fallback. Client-set cursor surfaces and named shapes
are still not rendered, so an app asking for an I-beam gets the arrow.

Lid switch. libinput switch events are handled and surfaced to config as
srd.on("lid_closed"/"lid_open", fn), so closing the lid can lock and suspend
instead of doing nothing.

Config-driven additions, each needed by a binding in the ported config and
none of which existed: fullscreen (Window.fullscreen was a dead field --
declared, never read or written), directional window move that swaps with
the neighbour and reorders the stack so tiling follows, focus cycling,
modifier+drag to move/resize anywhere in a window rather than only by the
titlebar, modifier+scroll to change workspace, and 8 XF86 media/power
keysyms taken from the system's own XF86keysym.h. The keysym tables are
hand-maintained in both directions and a key missing from either fails
silently, so a round-trip test now covers every one the configs bind.

Verified in the QEMU VM: a bare-TTY screendump shows a recognisable arrow at
the pointer position (113 white fill + 58 black outline pixels at screen
centre, where the pointer starts).

1 files changed, 171 insertions, 0 deletions

diff --git a/crates/wayland/src/cursor.rs b/crates/wayland/src/cursor.rs
new file mode 100644
index 0000000..121e8e1
--- /dev/null
+++ b/crates/wayland/src/cursor.rs
@@ -0,0 +1,171 @@
+//! Mouse cursor rendering.
+//!
+//! The nested (winit) backend gets a cursor for free - the *host*
+//! compositor draws one over srdwm's window - which is exactly why this was
+//! missing for so long without being noticed. On a bare TTY nothing else is
+//! drawing anything, so without this the pointer is simply invisible: you
+//! can move it, click with it, and drag windows with it, but you cannot see
+//! where it is. That makes the udev backend unusable as a real session.
+//!
+//! **Scope, stated plainly:** this draws one built-in arrow, always. It
+//! honours `CursorImageStatus::Hidden` (so a client that hides the pointer
+//! still gets its way), but it does *not* yet render a client's own cursor
+//! surface or a named shape - an app asking for an I-beam or a resize arrow
+//! still sees this arrow. That is a real limitation, and a visible one over
+//! text fields; it is also strictly better than the previous behaviour of
+//! drawing nothing at all.
+//!
+//! The built-in arrow is deliberate rather than loading an XCursor theme:
+//! theme loading pulls in a dependency, needs a theme to actually be
+//! installed, and has a search-path fallback story of its own. A cursor that
+//! is always present beats a prettier one that sometimes isn't there - the
+//! same reasoning as `decoration.rs`'s font fallback.
+
+use smithay::utils::{Logical, Point};
+
+/// Side length of the built-in cursor bitmap, in pixels.
+pub(crate) const CURSOR_SIZE: i32 = 24;
+
+/// A classic left-pointing arrow: white fill, black outline, with the
+/// hotspot at (0, 0) - the tip.
+///
+/// Encoded as a small bitmap rather than drawn with geometry so the shape is
+/// obvious and reviewable: `.` transparent, `#` black outline, `*` white
+/// fill. 24 rows of 24 columns.
+const ARROW: [&str; CURSOR_SIZE as usize] = [
+    "#.......................",
+    "##......................",
+    "#*#.....................",
+    "#**#....................",
+    "#***#...................",
+    "#****#..................",
+    "#*****#.................",
+    "#******#................",
+    "#*******#...............",
+    "#********#..............",
+    "#*********#.............",
+    "#**********#............",
+    "#***********#...........",
+    "#************#..........",
+    "#*************#.........",
+    "#******####### .........",
+    "#***#**#................",
+    "#**#.#**#...............",
+    "#*#..#**#...............",
+    "##....#**#..............",
+    "#.....#**#..............",
+    ".......###..............",
+    "........................",
+    "........................",
+];
+
+/// Rasterises the built-in arrow as premultiplied ARGB8888, the format
+/// `MemoryRenderBuffer` expects.
+pub(crate) fn arrow_bitmap() -> Vec<u8> {
+    let mut buf = vec![0u8; (CURSOR_SIZE * CURSOR_SIZE * 4) as usize];
+    for (y, row) in ARROW.iter().enumerate() {
+        for (x, ch) in row.chars().enumerate() {
+            if x >= CURSOR_SIZE as usize || y >= CURSOR_SIZE as usize {
+                break;
+            }
+            // Premultiplied: opaque pixels only, so colour == colour * 1.
+            let (b, g, r, a) = match ch {
+                '#' => (0x00, 0x00, 0x00, 0xff),
+                '*' => (0xff, 0xff, 0xff, 0xff),
+                _ => continue,
+            };
+            let i = (y * CURSOR_SIZE as usize + x) * 4;
+            buf[i] = b;
+            buf[i + 1] = g;
+            buf[i + 2] = r;
+            buf[i + 3] = a;
+        }
+    }
+    buf
+}
+
+
+/// Render elements for the pointer, to be drawn above everything else.
+///
+/// `pos` is in the global space and `origin` is the output's origin, since
+/// each head renders in its own coordinate space.
+///
+/// Returns nothing when the cursor is hidden, or when the pointer is not
+/// over this output - otherwise every monitor would draw its own copy.
+pub(crate) fn render_elements<R>(
+    status: &smithay::input::pointer::CursorImageStatus,
+    buffer: &smithay::backend::renderer::element::memory::MemoryRenderBuffer,
+    renderer: &mut R,
+    pos: Point<f64, Logical>,
+    origin: Point<i32, Logical>,
+    size: (i32, i32),
+) -> Vec<smithay::backend::renderer::element::memory::MemoryRenderBufferRenderElement<R>>
+where
+    R: smithay::backend::renderer::Renderer + smithay::backend::renderer::ImportMem,
+    R::TextureId: Clone + Send + 'static,
+{
+    use smithay::backend::renderer::element::memory::MemoryRenderBufferRenderElement;
+    use smithay::backend::renderer::element::Kind;
+    use smithay::input::pointer::CursorImageStatus;
+
+    if matches!(status, CursorImageStatus::Hidden) {
+        return Vec::new();
+    }
+    // Only the output the pointer is actually on draws it.
+    let local = (pos.x as i32 - origin.x, pos.y as i32 - origin.y);
+    if local.0 < 0 || local.1 < 0 || local.0 >= size.0 || local.1 >= size.1 {
+        return Vec::new();
+    }
+
+    // Built-in arrow: hotspot is the tip, so no offset.
+    let at = (local.0 as f64, local.1 as f64);
+    match MemoryRenderBufferRenderElement::from_buffer(renderer, at, buffer, None, None, None, Kind::Cursor) {
+        Ok(e) => vec![e],
+        Err(e) => {
+            log::warn!("cursor: failed to import bitmap: {e}");
+            Vec::new()
+        }
+    }
+}
+
+/// The built-in arrow as an uploadable buffer. Built once at startup rather
+/// than per frame - the bitmap never changes.
+pub(crate) fn make_buffer() -> smithay::backend::renderer::element::memory::MemoryRenderBuffer {
+    use smithay::backend::allocator::Fourcc;
+    use smithay::backend::renderer::element::memory::MemoryRenderBuffer;
+    use smithay::utils::Transform;
+    MemoryRenderBuffer::from_slice(&arrow_bitmap(), Fourcc::Argb8888, (CURSOR_SIZE, CURSOR_SIZE), 1, Transform::Normal, None)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn arrow_is_the_expected_size_and_has_an_opaque_tip() {
+        let buf = arrow_bitmap();
+        assert_eq!(buf.len(), (CURSOR_SIZE * CURSOR_SIZE * 4) as usize);
+        // The hotspot pixel (0,0) is the arrow's tip and must be visible,
+        // otherwise the cursor appears offset from where clicks land.
+        assert_eq!(buf[3], 0xff, "tip pixel must be opaque");
+    }
+
+    #[test]
+    fn arrow_has_both_outline_and_fill() {
+        let buf = arrow_bitmap();
+        let mut black = 0;
+        let mut white = 0;
+        for px in buf.chunks_exact(4) {
+            if px[3] == 0 {
+                continue;
+            }
+            if px[0] == 0 && px[1] == 0 && px[2] == 0 {
+                black += 1;
+            } else {
+                white += 1;
+            }
+        }
+        assert!(black > 20, "expected a black outline, got {black} px");
+        assert!(white > 40, "expected a white fill, got {white} px");
+    }
+}