https://bugs.winehq.org/show_bug.cgi?id=57661
--- Comment #2 from Rémi Bernon rbernon@codeweavers.com --- Created attachment 77840 --> https://bugs.winehq.org/attachment.cgi?id=77840 Fix
The issue is that there are two client windows created, one on the toplevel window, when D3D changes its pixel format, and another one in a child window that is used to render the video.
The toplevel window is created without WS_CLIPCHILDREN, so it doesn't actually need child window clipping, and the last window to present is supposed to be visible.
Originally the two client windows were put offscreen as there's a child window and we didn't bother checking the clipping rules, and the video child window was being blitted to the toplevel window itself through GDI blits.
Now, as we are checking WS_CLIPCHILDREN and the window region to decide whether we need to move the surface offscreen, it turns out that the toplevel client window doesn't need to and we're keeping it on screen. It however obscures its parent window pixels and any drawing that is done by other offscreen children.
The attached patch should fix the issue.