Regions that have auto-sizing don't recompute their size when the window resizes. This leads to content being clipped or overflowing (depending on the combination of region-overflow and overflow). Instead, the regions should resize, always keeping the content visible.
Created attachment 174143 [details]
HTML file highlighting the problem
Created attachment 174707 [details]
using contenteditable doens't expand region
This is very similar and most probably the same bug is behind it which is why I post it here. But instead of looking at window size changes, this is about changes the contents of the element, which should also make it auto-resize.
*** This bug has been marked as a duplicate of bug 102954 ***