bug#5117: 23.1.50; Weird display when wrap-prefix contains tabs

[Chong Yidong]

> > The "+ it->continuation_lines_width" was added for a reason.
>
> I tracked back through the history of the code, and that part has been
> there since the code was introduced as far as I can tell.
>
> > IIRC, it is so that continued lines containing tabs are displayed correctly.
>
> Can you define "correctly"?  When I remove the addition, it displays
> 'correctly' from what I expect: a tab always indents the text to the
> next visual column that's a multiple of 8.

Sorry for the delay in replying.

Suppose that (i) next tab stop is 3 characters off the right edge of the
text area, and (ii) you have text going all the way to the right edge.
Now insert a tab character.  With the current code, Emacs displays the
tab up to the 3rd character of the continuation line.  This is
"correct", in the sense that the continuation line is a "continuation"
of the previous line with the tab stops picking up where they left off.

With the patch you propose, Emacs would instead display up to the 8th
character of the continuation line.