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.