Since the libmpv bump to 20260809 (2.15.0), libmpv-2.dll dynamically links
the Khronos Vulkan loader instead of statically linking it. plezy.exe links
libmpv at load time, so on machines whose GPU driver does not install
vulkan-1.dll - Windows-on-ARM Adreno drivers, driverless x64 VMs - the
process failed before startup with "vulkan-1.dll was not found".
Download the LunarG-built loader (pinned, hash-verified) at configure time
and install it next to the executable with its license. Without a Vulkan
ICD the gpu-api=auto probe fails cleanly and falls back to D3D11, matching
the old static-loader behavior.
close#2110
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