This is a stripped down version of http://bugzilla.xfce.org/show_bug.cgi?id=4932

At the moment switching an app's window to fullscreen will remove all window decorations and render inaccessible all panels. If on the same virtual desktop two apps are in fullscreen mode, when switching between the two using <alt + tab> xfwm4 will unnecessarily briefly display the non-autohidden panels, creating a disturbing visual artefact. 

This could be solved by having an WM Tweaks option to set all panels to autohide when an app is fullscreen. This would allow to easily access the panels even when the app is in fullscreen mode. It would also ensure that the panels are not displayed when <alt + tab> switching between two fullscreen apps, effectively when working fullscreen with several apps on the same virtual desktop. 

Please let me know what you think