This is a minor, cosmetic bug that is simple to reproduce. Here's how:

1. Make a new document.
2. Turn on typewriter scrolling and selection highlighting.
3. Hit return.
4. Hit delete.

If this isn't somehow local to the two machines I've tried it on (both Intel macs running 10.6), then you should now see a residual horizontal line below the current (highlighted) line. Delete multiple lines of text, and you'll get a residual line for each text line you delete.

This only happens when you delete the last line of the document; all other lines redraw correctly.

HTH. Let me know if you need more info.