Comment 28 for bug 1760818

I installed Lubuntu 18.04 LTS on VirtualBox, and I met similar issue. The text messages on gajim chat window were messy. GTK didn't seem to refresh the window and kept previous text overlapped on the window.

I found a discussion (in Chinese) here, they found an workaround solution - changing desktop theme:

https://www.ubuntu-tw.org/modules/newbb/viewtopic.php?topic_id=108158&forum=2&post_id=360960

My input method was changed to gcin. Other than that, LXDE related settings were unchanged after Lubuntu installation.

After using lxappearance to change theme to "Industrial", the symptom disappeared.