Menu body appears far away from menu entry

Sidenote: I cannot quite describe this, if you feel like to have a better title, please retitle. Also it is possible that this is not strictly gajim bug, but I cannot conjure up good search keywords, and help is appreciated.

Versions

• OS: Linux Debian/sid
• Gajim version: 0.98.2
• GTK version: 3.22.26
• Python-nbxmpp version: 0.6.1
• WM: Xfce4 + fvwm4

Steps to reproduce the problem

1. open a menu ("Gajim"), possibly on HiDPI display (160 dpi)
2. the menu appears half a screen left from the gajim window.

Expected behavior

The menu opens below the menu seelctor, onto the Gajim window.

Sidenote

It started when Gajim was updated not to use the window managers default decoration but some absolutely useless one, missing buttons, having wrong size and fonts, etc. Maybe it's using GTK3, maybe else. Same effect was observable on Evince last year, but it got fixed up since; now it seems it's Gajim's turn. Since this is the case, and since I have acquired HiDPI monitor all Gajim menus open half a screen left. If it's not Gajim fault (apart from having bad window decorations, which definitely is) I'd really appreciate a pointer where to go and report it.

I attach a visual illustration of the problem: