Randomness - disappeared browser window reappears two days later.

Do you use the "Restore Previous Session" feature of your web browser between reboots? Do you use keyboard controls to open new windows like Ctrl+N?

If yes to both, chances are that your mystery window had remained part of that continued session, and was reactivated with a Ctrl+Shift+N or equivalent. Shift is right next to Ctrl so if you fat-fingered a Shift at the same time that could explain the apparent necromancy.

As to where it went originally, you could have moved it to a different virtual desktop, moved it off-screen accidentally or, heck, even misclicked or miskeyed to have it actually close. A reboot would then have closed it for certain, except for the fact that it was still lurking in your session.

Another thing that can cause windows to move around is when a HDMI or DisplayPort monitor disconnects. Unlike with older connection types, those disconnecting is detectable and modern window managers try to "save" windows by throwing them on any remaining active monitors. If you have both types of monitor connection, or had at some point in the past, it could be that your mysterious window was lurking on a screen you couldn't see but the system still thought exists, at least until it was closed for certain.

Admittedly unlikely, but you could rule that out by checking the system monitor arrangements in the settings.

If you use Firecat, they do that from time to time. You might want to put a location chip on them to know where they go. Careful they may bring some dead Firemice windows back on your screen.

Good ideas to investigate. Thanks for the suggestions.

At time of disappear, the keyboard was turned off (Bluetooth, I was watching videos) Only mouse was in use, and the focus was on a non streaming browser. (Disappeared streaming browser was on monitor two, steam and other items that disappeared were on primary monitor) I've since wondered if I somehow hit just the right combo of buttons on the mouse to cause it, but no luck reproducing.

I don't have restore previous sessions turned on. The haunted browser came back after a full day of normal use and two reboots. Having it reappear at the point in the video I'd been watching (and have since finished) really made me double take.

I use display port for both monitors, and have since installing this OS. Cables have not been reseated since then. I did have trouble getting both working correctly, (Nvidia card) so will double check that I don't have a ghostly monitor installed to the system.