Filed as FB25396997, with a sysdiagnose captured while the app was frozen. Additional data point requested above: I re-ran the same test with D3DM_MTL4=0 (Metal 3 backend) and the window freezes in exactly the same way — the process stays alive (~5-6% CPU) but no new frames are presented, and it never recovers. So the hang does not depend on the Metal 4 backend. Attached to the Feedback report: full Wine/UE log of the frozen run, UE client log, a sample taken while frozen, and the sysdiagnose. One remaining question: in an older build the log contained [D3DMetal:FATAL] Failed to get drawable for HWND. Run with D3DM_NO_WINDOW=1 env. but D3DM_NO_WINDOW is not in the GPTK 4.0b3 documented environment variables. Could you confirm whether it is a supported workaround for an occluded window, or whether it is only intended for headless use? Thank you.