wrong cursor position reported when window margins are wide

[Joost Kremers]

Hi all,

I recently had a bug report for a package[1] I maintain, which 
seems to be a bug in Emacs itself. The user that reported the bug 
uses Fcitx[2], an X-Window based input framework for Chinese 
(probably other Asian languages as well, but I have no expertise 
in that area). Another user reported the same problem for another 
such input framework, ibus[3].

Basically, these input methods consist of a small box (GUI window) 
that follows the text cursor. They rely on the underlying program 
(i.e., in this case Emacs) to report the cursor position in order 
to position the input box correctly.

It seems that when the window margins of an Emacs window are 
non-nil, the cursor position is reported incorrectly, resulting in 
the input box being displayed in the wrong place.

The original bug report is here:

<https://github.com/joostkremers/visual-fill-column/issues/25#issuecomment-281844388>

There are screen shots that illustrate the problem. (Scroll down a 
bit for screen shots with `emacs -Q'.)

The problem exists with Emacs v. 25.1.1, GTK+ for Linux. Emacs for 
Windows does not display a similar problem. (Which is the main 
reason why I think this is an Emacs bug rather than a bug in my 
package. Basically, all my package does is set the window margins 
to a non-nil value.)

Since I don't use ibus, Fcitx or any similar package, I cannot 
confirm the problem myself. Therefore, I figured it would be best 
to post here first before filing a bug report. I'm hoping that 
someone here can confirm the bug and perhaps tell me what exactly 
to put in the bug report to make it concise and to the point.

TIA

Joost





[1] https://github.com/joostkremers/visual-fill-column

[2] https://fcitx-im.org/wiki/Fcitx

[3] https://github.com/ibus/ibus/wiki

--
Joost Kremers
Life has its moments