[CSS Regions] Positioned elements in regions get clipped if they fall outside the region

This tests that overflowing borders are properly painted for sticky-positioned elements. The test passes if all borders are completely visible and the text THE END is visible

Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. THE END.