It's not a historic artefact. You can't trust the app to render in time... (that's also why client side decorations are worthless).
Re:Wayland in Gnome: two progress reports
by alexlarsson on Tuesday August 26, 2014 @03:40AM
(#8226090)
Well, even if you assume the app will take too long to render, is it really worthwhile to render the "rest" of the window (pointer, frames, etc). The main reason you're working with a particular window is the content, not the chrome, so drawing the chrome asynchronously only serves to make things look weird, not actually get things done faster.
I assume that window contents are drawn to a buffer before being rendered on a modern system anyway. I can't remember the last time I saw partially drawn window contents.