Fix: Jumping Panels

Short description of error
Panels are jumping strangely, caused by the scroller area which gets only correctly initialized when the mouse hovers a button in the region containing the panels.

Exact steps for others to reproduce the error
You can reproduce this for example by changing the outliner (default view) to the properties space. The scroller overflows the panels until you hover a button. This causes a jumping of the panels.

About the fix
This fix makes sure that the panel region gets initialized with the corresponding scrollbars. Well, this is only a fix for vertical aligned panels as a fix horizontal aligned panels would require to change the initialization for almost all regions containing panels individually. IMHO not worth doing since horizontal alignment isn't well-supported (yet).