This setting will be removed, I had already removed it from the navigation but must've forgotten the settings block...
Back when it was first implemented this setting allowed the settings and navigation blocks to double in size when the mouse was over them. Unfortunately the change to floating div layout rendered this setting unusable due to the newly introduced overflow settings trimming the content on expansion unless the block was specially lifted out of the page flow.
I've created MDL-23365 to ensure I do not forget it again
Cheers
Sam