Solved Triple Monitors - Something Changed?

I have used triple monitors with Win 11 for a long time.

I will run A or B+C with the other(s) being switched off.

Monitor A is DP. Monitor B+C are DP and HDMI.

When A is on it is the primary monitor. When B+C are on, B is the primary monitor.

This has worked flawlessly with Win 11 since I first started using 11. but now something has changed.

Now, if I power up with monitor C off then everything is fine. But the moment I do anything that involves C being on, the trouble starts, in that Windows seems not to know when C is subsequently switched off. The problem with this is that Windows will often open apps on a monitor that is switched off. To make matters worse, Windows sometimes gets confused enough to put up the wrong size wallpaper on monitor A (monitor C is much smaller than monitor A).

This has changed in the last month or so. Does anyone know what has happened?

I suppose the answer would be to use a DP monitor instead of an HDMI for C, but it doesn't have a DP connection.

Pocah said:

Does anyone know what has happened?

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Thank you for the reply!
Yes, everything is as it should be.
At the moment I am considering buying a DP monitor or just living with the problem.

Perhaps the most annoying thing is when Settings appears off-screen. You can't "Move" a settings window so the only thing to do is reboot the PC.

the settings window will open on the same screen it was closed on.

The curious thing is that the settings reset to the primary display if Windows boots with all other displays off. But if you turn off an hdmi, it still goes to that screen if it was on it before. And unlike other apps, you can't "move" a settings screen from the taskbar. How odd.
The big mystery for me is why can windows tell if a DP monitor is switched off, but it can't tell if an hdmi is? It knows whether one is on or off at power-up, so why not all the time? Rhetorical questions, I suppose, since it's not about to change all of a sudden. But what has changed is the odd behavior of the wallpaper.

In the Bios, the HDMI port is the default monitor input. It will always try to boot to that screen first.

I found it!!
Windows 11 remembers monitor configurations. So if you turn one off, then it adopts the remembered configuration for the ones that are on.
However, for reasons unknown, it managed to mess up. When I had B+C on and A off, it had B as the primary, but when I switched B+C off and A on, it set the primary to C!!! Of course it should have been A. So switching on C+A, I could access the settings and change the primary back to it's correct setting, A. Problems solved.

I eventually managed to resolve all the problems with an additional app.
Windows was working OK, except when I went from B+C to A., Windows still thought that C was on.
Also, I have a long standing issue that I need all the taskbars to be identical and in Win 11 they are things missing on the secondary monitor.
I solved all of the remaining problems with with something called DisplayFusion.
It duplicated the taskbars, which was great, but unexpectedly it sorted the other problem too.
You can disable a monitor with it, so when A was on and windows mistakenly thought C was too, I disabled C using DisplayFusion.
The interesting thing is that if I switch to B+C, Display Fusion seems to recognise this is not the same situation and re-enables the monitor. It disables it again when I go back to just A powered on!!!