JabRef 5.0-beta.362--2020-01-25--ac90ad2
Linux 5.4.13-3-manjaro amd64
Java 13.0.2
Steps to reproduce the behavior:

Expected behavior:
Actual behavior:
Perhaps this is the same but if you left click on a menu and try to scroll down to menu items they are not highlughted and cannot be selected. However, if you repeat the same
thing the second time the menu items become selectable.
I don't know if this is related, but when I open a menu from the menu bar, there is a gap between the menu bar and the menu. I don't know if that's the default behavior of the application, or if this is not supposed to happen.

Can confirm that the problem still persisits in Manjaro Linux, i3 wm, seems to be related to i3 wm issue. The problem got solved when I switched to xfce4 wm, where the menus in the menu bar became clickable again.
If it is relevant, I also did my tests on a tiling window manager (bspwm), so it may be a problem on these kinds of WMs...
I am on KDE Plasma 5
Maybe this is related to this issue? #5526
I am not sure, as we are dealing with floating menus here and not dialog windows.
But maybe fixing #5526 also fixes this one, who knows.
I'm also experiencing this under i3wm.
Having the same issue in i3wm.
Having the same issue in i3wm also
same issue with i3wm
I searched around and it seems to be an issue in the javafx version 14:
https://bugs.openjdk.java.net/browse/JDK-8251240
Based on the javafx issue cited above, running with environment variable GDK_DISPLAY=1 fixes this issue under XMonad (another tiling window manager).
Based on the javafx issue cited above, running with environment variable GDK_DISPLAY=1 fixes this issue under XMonad (another tiling window manager).
This seemed to work yesterday, but I cannot replicate the workaround today. Not sure what changed, please disregard.
However, here is another workaround that seems to work and might help diagnose the issue. Click and hold on menu (e.g. "file"), drag cursor out of JabRef window (e.g., into neighbouring tiled window, status bar, etc.). Release and menu stays open.
Alt then cursor keys also work for navigating the menus, but this is not a fix.
Most helpful comment
Can confirm that the problem still persisits in Manjaro Linux, i3 wm, seems to be related to i3 wm issue. The problem got solved when I switched to xfce4 wm, where the menus in the menu bar became clickable again.