Start GIMP and open a new image.
Set the mouse pointer to the top ruler above the image, keep the left mouse button pressed and drag a guide line to the image area.
When using XFWM nothing happens. Even more worse is that the window does not reply any more. You cannot select any menu item or drag the whole window. You also cannot select any tool from the toolbox window.
Using "openbox --replace" and restart gimp will fix that. When using KDE this isn't a problem either.
When using KDE and 'xfwm4 --replace' GIMP also will work fine so i'm not sure if this problem is related to xfwm4 only.
Platform: Slackware Linux 13.37 with XFCE 4.8 build from the source tarballs.
Works just fine here, tested with both xfce 4.6 and xfce 4.8 and gimp 2.6.11 (latest stable release)
Some other similar reports without xfce being involved:
More similar reports:
Not an xfce problem, closing as "worksforme"