The window geometry is just the logical bounding rect that the compositor should use to offset/size the window, so the extra space added by the shadows can get subtracted from that calculation. If there is rounded corners (or any other transparent regions), then a separate alpha-only texture would be needed to determine the graphical region that needs to be masked and blurred to draw the shadow. Though I guess that could be generated by the compositor by only using the alpha channel on the window surface.
I don’t see why anyone would ever do that, if they are following the HIG. That is the only place that button goes.
But it seems that whatever new thing would need to be exposed as a public property anyway if AdwWindowDecoration, AdwWindowTitle etc. are going to hook into it.