Problem: a graphics driver that crashes inside wgpu instance or device
creation (Intel Vulkan igvk64.dll #512#413#793, AMD atio6axx.dll #477)
killed every launch the same way until the user set WGPU_BACKEND by hand;
a start-up error wgpu did report ended in an error dialog.
Fix: each start leaves a marker in the data folder (graphics-start.json),
cleared once the window has painted two frames. A marker still pending at
the next launch moves on along the platform's ladder (Windows: Vulkan+DX12,
DX12 only, software/WARP; Linux: default, software/lavapipe or llvmpipe)
and keeps the mode that works. A wgpu start-up error moves on within the
same process (eframe reuses its event loop). Fallback modes don't register
the GPU export renderer, whose own instance would load the failed driver.
An explicit WGPU_BACKEND leaves the marker alone.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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