Transparent windows

[Vincent Povirk]

It is probably a layered window, in which case the following functions
in user32 are relevant: SetLayeredWindowAttributes,
GetLayeredWindowAttributes, UpdateLayeredWindowIndirect, and
UpdateLayeredWindow

Most of user32 is implemented based on gdi32, wineserver, and
winex11.drv. When you see USER_Driver in user32, that indicates a
function in winex11.drv. So USER_Driver->pSetLayeredWindowAttributes
is really X11DRV_SetLayeredWindowAttributes.

Wine requires a compositing window manager to implement layered
windows properly. It does not matter whether the window manager
provides any 3D effects or uses opengl.