I’ve been using fvwm2 for many years, mostly in happy ignorance - I’ve barely touched my .rc in that time through various hardware and (Linux) OS changes, and FocusFollowsMouse has been a primary part of my workflow in that time.
I’ve just started a new job in which the use of IntelliJ Idea  is highly recommended as a development environment, but I find that when I move my mouse back over its window it fails to acquire the focus. That’s going to make it rather hard for me to use effectively.
Because I don’t know what I’m doing with window-related things, I’m not sure how to investigate that further - is it likely to be a bug in Idea, a bug in FVWM2, or some altogether more complicated interaction?
I’m on Ubuntu 14.04, with standard package fvwm v. 2.6.5.ds-3, and (install from tarball) idea-IC-141.1532.4; my .fvwm2rc is at .
Any suggestions for where to start gratefully received.