Linux: draw on the desktop's GPU, not always the discrete one (#164)

* Linux: draw on the desktop's GPU, not always the discrete one

egui asks wgpu for the high-performance adapter. On a hybrid laptop in
PRIME on-demand mode (Intel + NVIDIA, X11, GNOME) that is the NVIDIA GPU
while the display server runs on the Intel one, so every frame is copied
between the GPUs. During a resize the window then shows stale or garbled
frames and can stop drawing until it is resized again.

On Linux and the BSDs, ask for no power preference instead: wgpu then
takes the first adapter Vulkan lists, and Mesa's device-select layer lists
the display server's GPU first (or the one DRI_PRIME picks). Measured on
an RTX 4060 + Intel RPL-S laptop: in on-demand mode the list starts with
the Intel GPU, in `prime-select nvidia` mode with the NVIDIA one, so the
launcher draws on whichever GPU drives the screens. WGPU_POWER_PREF still
overrides it. Windows and macOS keep their current choice.

Refs #69

* Linux GPU: keep wgpu's adapter order, but a software rasterizer last

Without Mesa's device-select layer the Vulkan order follows the ICD files, and lavapipe can come
before the real driver.

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

---------

Co-authored-by: Brandon Thomas <echelon@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

apps/artcraft-launcher/src/main.rs

@@ -218,7 +218,38 @@ fn adapter_rank(backend: eframe::wgpu::Backend, device_type: eframe::wgpu::Devic
     (backend != Backend::Dx12, device)
 }
 
-#[cfg(not(windows))]
+/// wgpu's adapter on Linux and the BSDs: the GPU the desktop runs on. egui asks for the
+/// high-performance GPU, which on a hybrid laptop in PRIME on-demand mode is the discrete one while
+/// the display server runs on the integrated one: every frame then crosses between the GPUs, and
+/// while the window is resized it shows stale or garbled frames and can stop drawing (issue #69).
+/// With no preference wgpu takes the first adapter Vulkan lists, and Mesa's device-select layer
+/// lists the display server's GPU first (or the one `DRI_PRIME` picks). Without that layer the
+/// order follows the ICD files, where lavapipe (`lvp_icd`) can sort before the real driver, so a
+/// software rasterizer only wins when nothing else can draw. `WGPU_POWER_PREF` (`low`, `high` or
+/// `none`) still overrides it, and then wgpu picks the adapter itself.
+#[cfg(not(any(windows, target_os = "macos")))]
+fn wgpu_options() -> eframe::WgpuConfiguration {
+    use eframe::wgpu::{DeviceType, PowerPreference};
+    let mut config = eframe::WgpuConfiguration::default();
+    if let eframe::egui_wgpu::WgpuSetup::CreateNew(setup) = &mut config.wgpu_setup {
+        if let Some(preference) = PowerPreference::from_env() {
+            setup.power_preference = preference;
+        } else {
+            setup.power_preference = PowerPreference::None;
+            setup.native_adapter_selector = Some(Arc::new(|adapters, surface| {
+                adapters
+                    .iter()
+                    .filter(|a| surface.is_none_or(|s| a.is_surface_supported(s)))
+                    .min_by_key(|a| a.get_info().device_type == DeviceType::Cpu)
+                    .cloned()
+                    .ok_or_else(|| "no graphics adapter can draw to the window".to_owned())
+            }));
+        }
+    }
+    config
+}
+
+#[cfg(target_os = "macos")]
 fn wgpu_options() -> eframe::WgpuConfiguration {
     eframe::WgpuConfiguration::default()
 }
@@ -970,4 +1001,17 @@ mod tests {
         assert!(adapter_rank(Backend::Dx12, discrete, false) < adapter_rank(Backend::Dx12, integrated, false));
         assert!(adapter_rank(Backend::Dx12, integrated, true) < adapter_rank(Backend::Dx12, discrete, true));
     }
+
+    #[cfg(not(any(windows, target_os = "macos")))]
+    #[test]
+    fn linux_draws_on_the_desktops_gpu_unless_told_otherwise() {
+        use eframe::wgpu::PowerPreference;
+        let config = wgpu_options();
+        let eframe::egui_wgpu::WgpuSetup::CreateNew(setup) = &config.wgpu_setup else {
+            panic!("the launcher creates its own wgpu instance");
+        };
+        // egui would ask for the discrete GPU: on a PRIME laptop that's not the desktop's (issue #69).
+        let expected = PowerPreference::from_env().unwrap_or(PowerPreference::None);
+        assert_eq!(setup.power_preference, expected);
+    }
 }