bug#19553: 25.0.50; vertical-motion returns erroneous positive value

[Dmitry Gutov]

1. Create an empty buffer.

2. Create an overlay at the end of it with a tricky after-string:

(setq my-ov (make-overlay (point-max) (point-max) nil t t))
(overlay-put my-ov 'after-string "~\n~\n~")

3. Move point to the end of the buffer (it will be rendered after the
display string; not sure if it's a bug).

4. Evaluate (vertical-motion 1). Point won't move (it has nowhere to
go), but the function call with return 2. Repeat indefinitely.

It's apparently an old bug, but it's been reported in
https://github.com/company-mode/company-mode/issues/274 two days ago,
because it causes an error in company-mode's popup rendering code when
used together with a new-ish package.

(I think I have a workaround, though, if this only happens at eob).

In GNU Emacs 25.0.50.25 (x86_64-unknown-linux-gnu, GTK+ Version 3.12.2)
 of 2015-01-05 on axl
Repository revision: 1ac42bca8886267dac029a2f19df1a43313e0d95
Windowing system distributor `The X.Org Foundation', version 11.0.11601901
System Description:     Ubuntu 14.10