In some cases panels without headers were drawn too wide in sidebars with region overlap. Instead of always using the maximum width the view allows, headerless panels now also use the width calculated in `panel_draw_width_from_max_width_get`. The function already returns the correct width in all cases and also takes care of insetting the panels, when their backdrop needs to be drawn, which is necessary for headerless panels, too, when there is region overlap. Reviewed By: Hans Goudey Differential Revision: http://developer.blender.org/D17194