Fix #545: Windows renders through DX12 before Vulkan

Windows lists each GPU under both backends, and Vulkan came first. On an
Intel GPU its driver made the whole window flicker black while the
pointer moved; the same machine was fine with WGPU_BACKEND=dx12 or gl.
adapter_order now ranks a GPU's DX12 adapter before its Vulkan one, after
the power preference, so the GPU chosen doesn't change, only how it is
driven. Vulkan stays the fallback (a failed DX12 start restarts on it)
and WGPU_BACKEND=vulkan still picks it. Elsewhere there is no DX12
adapter, so nothing changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/vectorcraft/src/gpu.rs

@@ -4,9 +4,9 @@
 //! The canvas is rasterized on the CPU; the GPU only composites it and draws the UI, so any
 //! adapter that can present to the window will do. [`adapter_order`] ranks the adapters: those that
 //! can't present to the window's surface never, then the one `WGPU_ADAPTER_NAME` names, hardware
-//! before software, the native backends (Vulkan, Metal, DX12) before OpenGL, and the power
-//! preference ([`power_preference`]) among the rest. eframe is handed [`selector`], which takes
-//! the first of that order.
+//! before software, the native backends (Vulkan, Metal, DX12) before OpenGL, the power preference
+//! ([`power_preference`]) among the rest, and a GPU's DX12 adapter before its Vulkan one (#545).
+//! eframe is handed [`selector`], which takes the first of that order.
 //!
 //! An adapter can report that it presents to the window and still fail once it does: on a hybrid
 //! Linux desktop under Wayland, the compositor runs on one GPU and may refuse the frame buffers
@@ -89,15 +89,17 @@ fn key(backend: Backend, vendor: u32, device: u32) -> String {
 
 /// The order to try `candidates` in (their indices): only those that present to the window and
 /// aren't in `skip`; the one whose name contains `named` (any case) first; hardware before
-/// software; native backends before OpenGL; then by `power`, keeping the system's order among
-/// equals (all of it for [`PowerPreference::None`]).
+/// software; native backends before OpenGL; then by `power`; then DX12 before Vulkan, keeping the
+/// system's order among equals (all of it for [`PowerPreference::None`]). Windows lists each GPU
+/// under both, and Intel's Vulkan driver made the whole window flicker black where DX12 and OpenGL
+/// didn't (#545); elsewhere there is no DX12 adapter, so the order is unchanged.
 pub fn adapter_order(candidates: &[Candidate], power: PowerPreference, named: Option<&str>, skip: &[String]) -> Vec<usize> {
     let named = named.map(str::to_lowercase).filter(|n| !n.is_empty());
     let mut order: Vec<(usize, &Candidate)> = candidates.iter().enumerate().filter(|(_, c)| c.presents && !skip.contains(&c.key)).collect();
     // Stable, so equals keep the system's order.
     order.sort_by_key(|(_, c)| {
         let unnamed = named.as_ref().is_some_and(|n| !c.name.to_lowercase().contains(n));
-        (unnamed, c.device_type == DeviceType::Cpu, c.backend == Backend::Gl, power_rank(c.device_type, power))
+        (unnamed, c.device_type == DeviceType::Cpu, c.backend == Backend::Gl, power_rank(c.device_type, power), c.backend == Backend::Vulkan)
     });
     order.into_iter().map(|(i, _)| i).collect()
 }
@@ -361,7 +363,7 @@ mod tests {
     }
 
     /// A Windows machine lists every GPU twice (Vulkan and DX12): power saving still takes the
-    /// integrated GPU first, as wgpu's own choice did (#306).
+    /// integrated GPU first, as wgpu's own choice did (#306), each GPU through DX12 first (#545).
     #[test]
     fn hybrid_laptop_on_windows_renders_on_the_integrated_gpu_with_power_saving() {
         let c = vec![
@@ -372,8 +374,25 @@ mod tests {
             gpu("Microsoft Basic Render Driver", Backend::Dx12, DeviceType::Cpu),
         ];
         let order = adapter_order(&c, PowerPreference::LowPower, None, &[]);
-        assert_eq!(order, [1, 3, 0, 2, 4]);
-        assert_eq!(adapter_order(&c, PowerPreference::HighPerformance, None, &[]), [0, 2, 1, 3, 4]);
+        assert_eq!(order, [3, 1, 2, 0, 4]);
+        assert_eq!(adapter_order(&c, PowerPreference::HighPerformance, None, &[]), [2, 0, 3, 1, 4]);
+    }
+
+    /// The machine of #545: one Intel GPU, whose Vulkan driver made the window flicker black.
+    /// DX12 comes first whatever the preference; Vulkan stays a fallback, and `WGPU_ADAPTER_NAME`
+    /// or a restart leaving DX12 out still reach it.
+    #[test]
+    fn a_windows_gpu_renders_through_dx12_before_vulkan() {
+        let c = vec![
+            gpu("Intel(R) Graphics", Backend::Vulkan, DeviceType::IntegratedGpu),
+            gpu("Intel(R) Graphics", Backend::Dx12, DeviceType::IntegratedGpu),
+            gpu("Microsoft Basic Render Driver", Backend::Dx12, DeviceType::Cpu),
+            gpu("Intel(R) Graphics", Backend::Gl, DeviceType::Other),
+        ];
+        for power in [PowerPreference::LowPower, PowerPreference::HighPerformance, PowerPreference::None] {
+            assert_eq!(adapter_order(&c, power, None, &[]), [1, 0, 3, 2], "{power:?}");
+        }
+        assert_eq!(adapter_order(&c, PowerPreference::LowPower, None, &[c[1].key.clone()]), [0, 3, 2], "DX12 failed: Vulkan next");
     }
 
     #[test]

docs/development.md

@@ -38,7 +38,7 @@ How the web shell (`apps/vectorcraft-web/src/web.rs`) differs from desktop:
 
 The canvas is rasterized on the CPU (`vectorcraft-render`, vello_cpu); the GPU (wgpu, through eframe) only composites the canvas texture and draws the UI, so any GPU that can show the window will do. The desktop app picks the window's adapter itself (`apps/vectorcraft/src/gpu.rs`, eframe's `native_adapter_selector`) and logs every adapter it found and the one it uses.
 
-- **Order:** adapters that report they can't present to the window are never tried; then the one `WGPU_ADAPTER_NAME` names; hardware before software (llvmpipe, WARP); the native backends (Vulkan, Metal, DX12) before OpenGL; then the power preference, keeping the system's order among equals.
+- **Order:** adapters that report they can't present to the window are never tried; then the one `WGPU_ADAPTER_NAME` names; hardware before software (llvmpipe, WARP); the native backends (Vulkan, Metal, DX12) before OpenGL; then the power preference; then, on Windows, where each GPU is listed under both, a GPU's DX12 adapter before its Vulkan one (Intel's Vulkan driver made the whole window flicker black, #545), keeping the system's order among equals. `WGPU_BACKEND=vulkan` still picks Vulkan.
 - **Preferences › Performance › Graphics Processor** (`gpuPreference`): `automatic` (the default), `lowPower` (Power Saving, the integrated GPU) or `highPerformance` (the discrete GPU). The adapter is chosen when the window opens, so a change applies after a restart.
   - **Automatic on Windows and macOS** is power saving: the system shows frames from any GPU, and presenting frames rendered on a discrete GPU through the integrated one made the window flicker on some hybrid laptops (#306).
   - **Automatic on Linux and the BSDs** keeps the system's order: Mesa's Vulkan device-select layer puts the GPU the desktop runs on first (the integrated one on hybrid laptops; `DRI_PRIME` and `MESA_VK_DEVICE_SELECT` steer it). A Wayland compositor may not accept frames from another GPU: on a desktop whose compositor ran on an NVIDIA GPU, rendering on the Ryzen's integrated GPU made KWin end the window's connection ("importing the supplied dmabufs failed") and 0.5.0 crashed at startup (#502).