emacs -q
    M-: (run-at-time nil 0.01
                     (lambda ()
                       (with-current-buffer (get-buffer-create "rms")
                         (with-silent-modifications
                           (dotimes (_ 10)
                             (insert (if (= (random 2) 1) "gpl" "fsf") "\n")))
                         (when (> (- (point-max) (point-min)) 100)
                           (delete-region (point-min) (+ (point-min) 100)))))) RET
    C-x 2
    C-x 3
    C-x b rms RET
    C-x o
    C-x b rms2 RET
    C-x o
    C-x b rms2 RET
    M-: (dotimes (i 1000)
          (insert (propertize (format "%3d%3d%3d" i i i)
                              'mouse-face 'highlight)
                  "\n")) RET
    # Move your mouse pointer around within the rms2 windows and the
    # modeline of the rms window.

I see two glitches with regard to mouse-face highlighting.  Either there
is no highlighting at all or the font does not switch back to the
regular one after leaving the connected highlight region.