I just ran into something rather odd: I have an application which requires backing store. 4.0.1 has backing store off by default.
You can turn on backing store by giving the x server the +bs option, or by putting option "backingstore" in the Screen section of XF86Config. This XF86Config option is undocumented, and the Xserver man page only documents the -bs option -- it doesn't mention the use of the "+" sign, nor does it mention that backing store is turned off by default. I don't know which is right -- the documentation or the implementation. -- Raul