Merge branch 'master' into worktree-spot-removal

# Conflicts:
#	docs/traceability.md

ui/dr-ui/src/lib.rs

@@ -741,6 +741,7 @@ enum PointsUpdate {
 ///
 /// `None` means develop is unavailable and the viewer falls back to embedded
 /// previews — the same degradation as a machine with no adapter at all.
+#[cfg(not(target_os = "android"))]
 fn shared_gpu() -> Option<dr_gpu::GpuContext> {
     let shared = match pollster::block_on(dr_gpu::GpuContext::new_shared()) {
         Ok(shared) => shared,
@@ -781,6 +782,45 @@ fn shared_gpu() -> Option<dr_gpu::GpuContext> {
     Some(ctx)
 }
 
+/// TRACES: FR-DSP-1 | AC-8
+/// Open the compute device on Android — and do **not** give it to Slint.
+///
+/// # Why this platform is different
+///
+/// The desktop version above exists so one `wgpu::Texture` can be written by
+/// the adjust pass and sampled by the compositor without a round-trip. That
+/// requires Slint to be drawing with wgpu, and on Android drawing with wgpu
+/// means drawing on wgpu's Vulkan swapchain — which hardcodes
+/// `preTransform = IDENTITY` (gfx-rs/wgpu#3345).
+///
+/// On a tablet whose panel is mounted landscape, portrait then presents an
+/// unrotated buffer, every present returns `VK_SUBOPTIMAL_KHR`, and the frames
+/// arrive torn. Measured on the device: portrait sits on `composition=CLIENT`
+/// with `bufferTransform=ROT_270`, landscape on `composition=DEVICE` with
+/// `ROT_180`, and only portrait tore. Taking Slint off wgpu — it then uses
+/// Skia over OpenGL, where the driver owns the rotation — fixed it.
+///
+/// # What it costs, and why that is the right trade here
+///
+/// Without a shared device the display path needs a readback:
+/// `AdjustPass::export_pixels` instead of `Image::try_from(texture)`. That is
+/// the transfer ARCH §6.1 and AC-8 exist to avoid, and it is still the better
+/// bargain on this platform — the alternative is not a faster develop view, it
+/// is a torn one, in the orientation a tablet is mostly held in.
+///
+/// The compute passes are untouched: demosaic and adjust still run on the GPU,
+/// on this device. Only the last hop to the screen goes through memory.
+#[cfg(target_os = "android")]
+fn shared_gpu() -> Option<dr_gpu::GpuContext> {
+    match pollster::block_on(dr_gpu::GpuContext::new_headless()) {
+        Ok(ctx) => Some(ctx),
+        Err(e) => {
+            log::warn!("no GPU for the develop passes: {e}");
+            None
+        }
+    }
+}
+
 /// TRACES: M-13 | M-14
 /// Build and run the viewer.
 pub fn run(paths: Vec<PathBuf>) -> Result<()> {