https://bugs.winehq.org/show_bug.cgi?id=41930
--- Comment #45 from Bryan Varner bryan@varnernet.com --- After adding a bunch of debugging to dibdrv_wglMakeCurrent(...),
I'm quite confident that this is an upstream OSMesa bug.
Complete with a comment contained within osmesa.c, https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/gallium/frontends/o...
The osmesa context is not copying the user-supplied pixel buffer into the osmesa color buffer when OSMesaMakeCurrent() is invoked. Resulting in an empty (memset to all 0s) buffer being used.
The color_buffer in osmesa is then copied back overtop the user supplied buffer during glFlush() https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/gallium/frontends/o.... This obliterates the previously drawn contents to the device independent bitmap context, replacing them with the offscreen GL contents, which didn't have the original buffer copied in prior to the drawing functions.
It's an OSMesa bug.