Since version 26782 of xfwm4 when using enable display compositing the screen sometimes is not immediately cleared from a previous window.
e.g. After login the panels autohide but are only partially cleared, when using gxine and switching from full screen to a smaller screen parts of the previous screen are not immediately cleared.
I am using Debian Sid on a laptop with Intel chip 82852/855GM Integrated Graphics, kernel 2.6.24-1-686.
I guess it's caused by your new 'asynchronous frames update'!
Jos van Wolput
Bug fixed in svn revision 26815!
fixed in svn revision 26815