Middle mouse click toggles between windows - how to disable?

It's the wired Logitech M500 (latest Modell that has the shortened "logi" print).

I uninstalled the mouse, then restarted (press alt+F4 multiple times until a window comes up, then choose "restart" in the list and press enter) and it was back to normal (same 2006 driver again), thanks.

But: I now know what's causing this (kind of but no idea how to fix it): The mouse has got an extra button below the scroll wheel that switches between smooth scrolling (the way the wheel "runs"/feels in the mouse, not in actual Windows) and something that feels like it stops regularly (like a gear, this is the one I'm using). If I press that button (gear -> smooth), then clicking the scroll wheel goes back to opening that weird toggle modus. A PC restart doesn't help at that point, you have to uninstall the mouse again.

I tested the mouse on the 20H2 PC again and pressing the button there doesn't mess anything up. I also tested the 20H2 PC's M500 (slightly older model with the full "logitech" print) on the 21H2 PC and it's also not causing the problem.

What is that "toggle window" mode even? What keyboard shortcut usually enables it and how do I fully disable it?

I can say from experience that holding down the scroll wheel to scroll faster is supported by Win 10 by default. I've got a colleague (also Win 10 21H2) who uses a really old mouse (probably 10+ years, can't even read the manufacturer's name anymore) and even that one does it. Another colleague (also 21H2) uses some no-name ergonomic model (one of those almost vertical ones) and it does it too.

As I said, I never installed any drivers or changed any settings on the 21H2 PC. I plugged in the mouse, that's it.

I just compared mouse drivers between both PCs in the device manager: Both mice are listed as "HID-compliant mouse" and they both use driver 10.0.19041.1 from June 21st 2006.

It looks like Win 10 messed it up on its own somehow and I don't want to install some additional software (that's not necessary on the other PCs) to fix it.

If a branded mouse, eg Logitech it will have its own driver/software

As far as I've ever known, Windows only offers scroll wheel settings, nothing for click or tilt, so that would still be a dedicated mouse software thing or another 3rd party thing such as a computer manufacturer software.

No one here knows what you have installed or not installed nor set or not set.

I'm sure you should be able to fix it by installing Logitech software and configuring the scroll wheel click to something else or disabled which I believe it will probably allow.

It's a Logitech M500 (wired). I'm using the same model on the 20H2 PC but I never installed any drivers/apps for it on either PC, so whatever there is Win 10 downloaded/installed on its own.