Since I updated Leap 15.1 -> 15.2, emacs is giving me a little problem under
Xfce:

dead keys[1] to type accented letters (eg: "ALT+` a" gives "���") stopped
working,
unless the environment variable XMODIFIERS is unset. After typing ALT+` emacs
now says "<dead-grave> is undefined".

For the records, echo $XMODIFIERS says "@im=ibus", and that apparently
interferes with emacs' handling of dead keys. Googling for this problem show
several hits[2][3][4], and more or less people seem content with launching

  XMODIFIERS= emacs

My problem with that is I don't launch emacs from the shell but use something
called xfce4-appfinder, i.e. a small text box that pops up when I do ALT+F2.

All in all this is more of a question than an actual bug report. How can I
restore the convenient behavior I used to get in Leap 15.1 / Xfce?

Version of stuff (what's in Leap 15.2):

* emacs 25.3.1
* Xfce 4.14

[1] https://en.wikipedia.org/wiki/Dead_key
[2] https://emacs.stackexchange.com/questions/38252/dead-grave-is-undefined
[3]
https://emacs.stackexchange.com/questions/2568/problem-dead-acute-is-undefined-appearing-when-typing-greek-stress
[4] https://bugs.launchpad.net/emacs-snapshot/+bug/1251176