So your current conclusion is simply get back to default settings in the Desktop Effect settings page? I’ll try that, for sure I have changed some settings so it may be worth giving it a try. Will update here in due time.
EDIT: so far, so good. I took screenshot of all my Desktop Effects settings so I will compare and try to troubleshoot it deeper
EDIT2: OK I compared default settings to my previous settings, I made this image to help me troubleshoot, green circle means I had the setting before, green strike means I had not the setting before.
After resetting settings to default, I tried each different setting one by one, by enabling and disabling the second monitor, and I couldn’t trigger the issue, even after setting everything back to where it was when I had the issue. Now it is like before, in theory, but there is no more issue of desktop effects being disabled when the secondary monitor is enabled or disabled.
I think you found the fix to that non sense issue, resetting to defaults (then in my case going back to all previous settings). It doesn’t really make sense to me but OK. Probably we had an error in a configuration file, and resetting to default fixed the error maybe?
Anyway problem seems solved for now
EDIT AFTER TOPIC CLOSED: and issue is still there, if I enable or disable secondary screen while watching a video in browser or external player.