I'm not sure of the trigger or culprit
but frequently enough one rolled up fence which is at the bottom right next windows taskbar (always the same fence)
will change its rolling "side"
I recently tried to fix it by using "lock fences" but it still bugged out.
What happens exactly :
-at start I setup the fence to have its titlebar right next to the windows taskbar & it develops upward
-after it bugged out, the titlebar of the fence is around the middle of the screen & develops downward toward the taskbar
It may happen after gaming & resolution changes, or sleep wake up.... no sure
OS: Windows 7 x64
Thanks for your attention.