I don't know if this is an unintentional side effect but the background element of a tab panel is the grey/white stipe pattern on OS X. If you place a tab panel and then delete all the tabs, you are left with a rectangle "object" that has this background topped by a blue bar. Overlay the blue bar with a special-matte border rectangle to make it go away and voila! -- a resizable rectangle with a grey/white stripe background. On a PC it comes out as a light grey background so no problems there.
I hope this trick doesn't break in future builds as I'm basing my design templates around this feature. Beats going to photoshop everytime I need a different size rectangle with this pattern fill...
- david