Unrelated IntelliJ windows brought foreward

I'm using IntelliJ 2017.3.2 on Ubuntu 17.10 (Gnome desktop), with two monitors.

Monitor 1: IntelliJ
Monitor 2: Firefox, also another IntelliJ window

Steps:

1. Open Firefox on monitor 2 (it's in the foreground now on this monitor, overlaying IntelliJ)
2. Open some kind of "modal window" in IntelliJ on monitor 1

Expected: "modal window" appears on monitor 1, nothing happens on monitor 2
Actual: "modal window" appears on monitor 1, on monitor 2 the other IntelliJ window is brought to the foreground (it's in the foreground now on this monitor, overlaying Firefox)

Is this a bug, or does anyone think this behavior is somehow configurable?

Admittedly it's a small nuisance, but it's just bothering me.

Thanks for any ideas.