Emacs 21.1 display bug

[Gustav Hållberg]

If you have wide tabs in a buffer (as in wider than your frame width),
the text doesn't display properly.

To repeat, start emacs 21.1 (with -q), and type (replace "100" with
something that's actually larger than your frame width, ^Q == C-q etc.):

M-x set-variable
tab-width
100
test^Q^Iing^Aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

and you should see some very weird behaviour of the display (the "ing"
part which is to the right of the tab will start moving left on the
screen while we're inserting stuff at the beginning of the line). 

- Gustav