I think this completely broke window switching on X11 when using the WM_TAKE_FOCUS protocol -which is the default.
We send this WM_MOUSEACTIVATE message in the handle_wm_protocols function in dlls/winex11.drv/event.c, to check whether a window should be activated whenever it is about to be focused, and it now always return MA_NOACTIVATE so we go through the fallback logic which is not correct in general -it only has a few heuristics and in general tries to focus the previously focused window.
I can confirm that the return value is correct though: on Windows clicking on the title bar triggers the WM_MOUSEACTIVATE message, which returns MA_NOACTIVATE, but then the window seems to be focused anyway.
Also, when switching windows with Alt-Tab for example the WM_MOUSEACTIVATE message is not involved on Windows, of course.