bug#23570: 25.1.50; Cursor motion error with visual-line-mode

[Chong Yidong]

When visual-line mode is enabled, there is a bug causing the cursor to
fail to find the right start/end of a screen line.  I see the bug with
Emacs 24.5, as well as the latest git version.  No bug in Emacs 24.4.

1. emacs -Q

2. Insert the following (at the start of a new line):

123456789 123456789 123456789 123456789 123456789 123456789 123456789 12 
ABCDEFGHIJK  MOPQ

3. M-x visual-line-mode RET

4. Adjust the window width until it is just large enough for
   "ABCDEFGHIJK" to be on the first line.

5. Decrease the window width by 1 character.  Now, "ABCDEFGHIJK" should
   be wrapped to the second line.

5. Move point to the beginning of the line starting with "1234...".
   Now, typing C-e moves to the second screen line.  Expected result: it
   should move to the end of the first screen line.

   Various other screen oddities are observable in the vicinity of this
   wrapped line.  For instance, when point is on "Q", C-a moves to the
   space before "M", rather than the start of the line.


In GNU Emacs 25.1.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.20.3)
 of 2016-05-17 built on tsparkle
Repository revision: 631ca55c6decccca2dc0961dc28962819eacc35b
Windowing system distributor 'The X.Org Foundation', version 11.0.11801000
System Description:     Arch Linux