Default Windows startup to DirectX 12 (#16)

eframe's default wgpu instance enables PRIMARY | GL on Windows. Creating
the OpenGL backend can crash inside AMD's atio6axx.dll before the window
appears, so the app exits silently at launch. When WGPU_BACKEND is not
set, the Windows instance now enables only DirectX 12; an explicit
WGPU_BACKEND is still honoured. macOS, Linux and web are unchanged.

Same fix as storytold/designcraft#168 and storytold/filmcraft#179.


Claude-Session: https://claude.ai/code/session_01LyVbT6Vv4BwWZ4hY8973qD

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

README.md

@@ -170,6 +170,21 @@ scorecard.
 
 Installers and executables are code-signed.
 
+**If the app doesn't open on Windows:** the desktop app initializes only DirectX 12 by default.
+Letting wgpu also create an OpenGL instance can crash some graphics drivers (AMD's
+`atio6axx.dll`) before the window appears, so the app would flash in Task Manager and quit.
+`WGPU_BACKEND` overrides the default for troubleshooting (for example `dx12` or `vulkan`). In
+PowerShell, from the folder containing the executable:
+
+```powershell
+$env:WGPU_BACKEND = "vulkan"
+& .\deckcraft.exe
+Remove-Item Env:WGPU_BACKEND                     # restore the default for later launches
+```
+
+An explicit `gl` override can bring the driver crash back on affected systems. The macOS, Linux
+and web backend defaults are unchanged.
+
 ### macOS
 
 | Build | File | Notes |

apps/deckcraft/src/graphics.rs

@@ -0,0 +1,47 @@
+//! Windows graphics instance configuration, applied before wgpu can initialize any driver.
+
+use eframe::{NativeOptions, egui_wgpu::WgpuSetup, wgpu::Backends};
+
+/// Restrict the wgpu instance to DirectX 12 unless `backend_override` (normally
+/// `Backends::from_env()`, i.e. `WGPU_BACKEND`) names other backends.
+pub fn configure(options: &mut NativeOptions, backend_override: Option<Backends>) {
+    if let WgpuSetup::CreateNew(create) = &mut options.wgpu_options.wgpu_setup {
+        // eframe's default includes GL. Creating that backend can crash inside AMD's
+        // atio6axx.dll before adapter selection or any Rust error handling runs, so the
+        // window never appears. Choosing an adapter afterwards is too late: only leaving GL
+        // out of the instance avoids it. This matches the verified WGPU_BACKEND=dx12
+        // workaround without mutating process environment variables.
+        create.instance_descriptor.backends = backend_override.unwrap_or(Backends::DX12);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn configured_backends(backend_override: Option<Backends>) -> Backends {
+        let mut options = NativeOptions::default();
+        configure(&mut options, backend_override);
+        let WgpuSetup::CreateNew(create) = options.wgpu_options.wgpu_setup else {
+            panic!("default native options must create a wgpu instance");
+        };
+        create.instance_descriptor.backends
+    }
+
+    #[test]
+    fn windows_default_initializes_only_dx12() {
+        // eframe's own default (PRIMARY | GL) would initialize OpenGL as well.
+        let WgpuSetup::CreateNew(eframe_default) = NativeOptions::default().wgpu_options.wgpu_setup else {
+            panic!("default native options must create a wgpu instance");
+        };
+        assert!(eframe_default.instance_descriptor.backends.contains(Backends::GL));
+        assert_eq!(configured_backends(None), Backends::DX12);
+    }
+
+    #[test]
+    fn explicit_backend_override_is_preserved() {
+        for backends in [Backends::VULKAN, Backends::GL, Backends::DX12, Backends::VULKAN | Backends::DX12, Backends::empty()] {
+            assert_eq!(configured_backends(Some(backends)), backends);
+        }
+    }
+}

apps/deckcraft/src/main.rs

@@ -14,6 +14,8 @@
 
 mod audio;
 mod control_server;
+#[cfg(any(target_os = "windows", test))]
+mod graphics;
 mod logging;
 
 use deckcraft_engine::Session;
@@ -188,6 +190,9 @@ fn main() -> eframe::Result {
             b.with_x11();
         }));
     }
+    // Before eframe creates the wgpu instance: default Windows to DirectX 12 only (see graphics.rs).
+    #[cfg(target_os = "windows")]
+    graphics::configure(&mut options, eframe::wgpu::Backends::from_env());
     eframe::run_native(
         "DeckCraft",
         options,