How do I find out why the render loop is running?

[Mike Hearn]

My app consistently uses about 10-15% of a CPU even when off screen and
nothing is being animated. I had assumed JFX would idle in this case but it
seems not. I suspect I've done something that is forcing the system to
"pulse" no matter what, but I don't know where to begin investigating this.
Profiling just shows lots of time being spent in quantum.