I'm using JetBrains Rider 2019.3.1 on Xubuntu 19.10 to develop a Gtk3 app using the GtkSharp wrapper v184.108.40.206 from NuGet.
When debugging in Rider, if I break whilst a GtkEntryCompletion popup is visible, all mouse clicks are swallowed on the whole XFCE desktop.
The mouse pointer moves as normal, but no clicks are registered anywhere in the OS, and no hover functionality works (tooltips etc.)
Luckily the keyboard still works, so I can Alt-Tab to Rider and then F5 to continue (then the mouse buttons come back to life).
Here is a test app that shows the issue: https://gist.github.com/awittaker/d7ffa77e30d1231a9fb800f07b0a5066, along with a typical idea.log.