@Stefan, thanks for the bug fix. I don't see this issue after updating to
this commit:
http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=944d77f070da388b0c6e6578a9f868e88c088940

On Thu, Sep 3, 2015 at 6:31 PM Katsumi Yamaoka <yamaoka@jpl.org> wrote:

> On Thu, 03 Sep 2015 19:39:36 +0300, Eli Zaretskii wrote:
> > FWIW, I cannot reproduce this with the current master.
>
> I've seen it a few times yesterday.  I'll report it if I find out
> a generic way to reproduce it.
>
> In GNU Emacs 25.0.50.1 (i686-pc-cygwin, GTK+ Version 3.10.9)
>  of 2015-09-03
> Windowing system distributor 'The Cygwin/X Project', version 11.0.11501000
> Configured using:
>  'configure --verbose --with-x-toolkit=gtk3'
>