Dual monitor setup shows inactive screen as black rectangle at startup since update 20.04-22.04

[udippel]

Setup; unchanged since 20.04 (where it worked):
Laptop, screen 1920x1080, not enabled, set in configuration
Monitor, screen 2540x1440, set as enabled, primary

After start, login, desktop coming up, a black rectangle of the size of the disabled laptop screen appears in the upper left corner, covering the larger part of the desktop. After some 30 seconds, it disappears on its own.

This happens since my upgrade 20.04->22.04; without any changes on my side w.r.t. hardware or settings.

It doesn't damage anything, it is just a bore. But one that indicates an underlying problem: the disabled screen doesn't have to be laid over the enabled screen for some time. It should be off.