Sticky panel

Verify the behavior of the sticky panel inside the overflowing (scrollable) containers. Best to run with --debug=stickypanel flag.

Tests:

• Panel should stick only when it's active.
• Panel should not change its state when scrolling the element that isn't its ancestor (drawn rects will disappear though).
• Sticky toolbar inside the inner container.
• Sticking the toolbar when scrolling the outer container.
• Top viewport offset should be taken into account when scrolling the window.
• Panel should stick to the bottom of the container when it's near end.