In the Mus window server, window decorations and the client area are different sibling Surfaces. It is conceivable that during resize, the client area would update faster than the window decorations (although maybe unlikely?). This seems undesirable for two reasons:

1. The client area would stick outside the bounds of the window decorations if we don't clip (icky). This might happen if the client isn't producing frames fast enough or today, if the client area is producing frames faster than the window manager (unlikely and can be mitigated if we throttle the client area on the window manager).

2. If we clip the client area then again, the user will see the jarring effect of one part of the window change while another does not.

There is a surface lifetime management issue here. We need to preserve old surfaces during a resize such that the client area never updates faster than window decorations.