This is so trivial, that I wasn’t really sure whether I should file a bug report. Also, it’s happening in Lubuntu 19.04, so it’s always possible that it could have been fixed upstream already.
I have an unusual setup where I have 2 task managers on my left and right screen. I have both configured to not share tasks, which means I can chuck background tasks like chat windows etc. on the other screen (which is very handy).
I noticed that if you move Firefox to another screen, and then minimise it, the minimise animation happens on the wrong screen. On subsequent times, when you maximise it and minimise it again (without moving the window), the animation is then correct.
I also encountered some other oddness when trying to reproduce this. I tried with my home directory (pcmanfm-qt), which opens right at the top-left edge of my 2nd screen (on the right). If I minimise straight away, without performing any other actions, it gets lost in a task manager ether, and appears in neither of my task managers. Once I open a second pcmanfm-qt window, I then see both. I see the same behaviour no matter how many pcmanfm-qt windows I already have open. If I move the window before minimising it, it does not get lost; however it does always minimise to the same task manager (no matter which screen it’s being minimised from).
Is it worth filing a bug report? It’s a lot of weird subtle behaviour that’s difficult to describe, so it would take me some time to properly test it and make sure it’s reproducible, hence why I wanted to post it on the forum first.
It would be useful to test this with different multiple monitor configurations, e.g. the primary screen on the left and right, and same/different resolution screens.