If content flowed in a fixed size region has the contentEditable attribute set to true, repaint artifacts will be visible if the edited content overflows the region.
See attached file for a repro and screenshot for actual results.
Created attachment 209184 [details]
HTML file highlighting the problem
1. start inputting text until you have at least 4-5 lines outside the region (black border)
2. Move the caret through the overflowing text using left/right arrows
Moving the mouse caret erases part of the lines
Created attachment 209185 [details]
Screenshot of the bug in action :)
See the second line after the bottom border - parts of the text is missing due to the caret moving through it.
This was fixed as a part of the visual overflow effort. Closing.