Resizing windows with all but the very latest NVIDIA drivers on Windows 7 64-bit causes the NVIDIA OpenGL driver to crash (particularly when combined with an immediate filled rectangle), so perhaps something sketchy is happening behind the scenes. Anyway, if somebody else has a similar problem, updating to the latest drivers might be the fix. (As it was for me.)

Also, when resizing with the D3D driver, no event is triggered until the mouse is released. So if you hold and wait, you are just left with some ugliness in the new space. I prefer the OpenGL driver's method (even if it does make it slightly more laggy) of triggering multiple events throughout the resize process.