to see the bug:
1. Enable wrap-by-window mode
2. Exit EmEditor
3. Start EmEditor, open a file with long lines – so that previously set Wrap-by-window mode is used
4. Scroll a window by vertical scrollbar up and down many times – BUG: parts of text multiplied all over the window
5. Switch to another application (AltTAB) that overlaps the EmEditor window, then back to EmEditor – BUG: EmEditor’s window will contain parts of the other app
p.s. Bug is avoided by: using Nowrap mode, using Wrap-by-margin mode, switching Wrap mode after opening a file, resizing a window