bug#489: Mouse highlighting bug

[Chong Yidong]

There seems to be a bug is how mouse-face highlighting handles extending
faces to the end of the line:

(global-set-key [f1]
  (lambda () (interactive)
    (switch-to-buffer "asdf")
    (erase-buffer)
    (insert "123456\n890")
    (put-text-property 4 9 'mouse-face 'highlight)
    (put-text-property 4 9 'face (list :underline t))))

Put the mouse over the region with mouse-highlight, then remove it.
Result: the space after the first line becomes underlined.