Created attachment 114545 [details] The black box This is possibly related to bug #311759. 1. Install `vim-gnome`. 2. Start `gvim`. 3. Write `:set ballooneval balloonexpr='hello'` 4. Add a line to the file. 5. Hover your mouse over the line in the file. You ought to see a balloon over the text with the message "hello." With `vim-gnome` in KDE Neon, you will see a black box with no text inside it instead. I suspect this has something to do with KDE support for GNOME applications. I've attached a screenshot showing what I'm talking about.
The issue does go away if I change the GTK3 theme from "Breeze" to "Breeze-Dark" in System Settings. I suspect there might be an issue with the default GTK3 theme.
*** This bug has been marked as a duplicate of bug 355540 ***