I came across a small bug when trying to figure out why pswp__ui--idle was not triggering consistently. Turns out it it does not trigger if options.mouseUsed = true has been force-fed from the options upfront.
options.mouseUsed = true
See example below. I just added mouseUsed: true, so make sure to test it from a mouse device. As you will see, the interface never hides.
Obviously, the simple solution is to not use this option at all, and just let Photoswipe detect mouseUsed. Just wanted to report.