Display bug with miscomputed window-start

[Andreas Schwab <schwab@suse.de>]

Redisplay does not correctly handle characters that wrap around at the
right margin.  To reproduce:

- Insert a bunch of ^A (C-q C-a) in a buffer so that the line wraps around
- Go to point-min
- If there is an even number of columns insert a space, so that the last
  ^A in the line wraps around, i.e the ^ is in the last column and the A
  appears at the start of the next screen line.
- Do M-1 C-v (scroll-up 1)

The cursor will now be somewhere in the middle of the window, maybe not
even inside any buffer text.  Now click on first visible ^A to move point
there and evaluate (list (point) (window-start)).  You will get two
different numbers that are off by one (`(42 41)' for an 80 column
display).

Note that if you change the character that wraps around to a TAB this bug
does not occur.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux AG, Deutschherrnstr. 15-19, D-90429 Nürnberg
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