Tab characters and wide runes at the right edge of a window overwrite the UI

[dmaluka]

Description of the problem or steps to reproduce

1. Disable tabstospaces.
2. Type a line containing a tab character.
3. Move the cursor to the beginning of the line.
4. Open a vertical split window on the right with an empty file.
5. Drag the vertical divider to the left, so that it's in the middle of the tab character.
6. Select the line.

The tab character overwrites the adjacent window:

Expected behavior: display only as many of the tab's spaces as fit in the buffer window. Also, if softwrap is on, maybe it makes sense to display the remaining spaces in the next visual line.

Also a similar issue is observed with wide runes. If a wide rune at the right edge doesn't fit in the window, it overwrites the divider:

Expected behavior: if a wide rune doesn't fit, it is not displayed in this visual line at all. If softwrap is on, it is displayed in the next visual line.

Specifications

Commit hash: 84f7248
OS: any
Terminal: any