Fix overflow when scrolling child widgets

[docelic]

Run test/widget-scrollable-boxes in a small window.
Scroll down using down arrow until the yellow elements comes into view.
Keep scrolling and notice how the element is properly contained within the parent (overflow is hidden).

Keep scrolling until the border of the yellow element reaches the border of the parent's parent (i.e. when the absolute y of the green box and yellow box becomes equal).
At that moment, the overflow prevention will stop working correctly and the yellow widget will render outside of parent (purple box) boundaries.

Investigate why and fix.

(Same issue is present in Blessed.)