How to prevent macOS From Switching Apps to Different Spaces?

Question

I like creating multiple Spaces on macOS (like different Desktops on Linux). I also like to keep apps in their own spaces: Browser in desktop 1, editor in 2, and email app in 3.

So I create 3 spaces by swiping 4 fingers up, then click + button to add spaces.

Then in first space I right click on the Dock icon of my browser and select “Options > This Desktop.” I switch to the other Spaces and do the same thing for the other apps.

Everything is working fine, until some time later I notice that browser is now in desktop 3, editor 1, and email in 2. The Dock options menu shows that the selections have also changed.

How can I make the options stick?

Answer 1

Copied directly from my existing answer at https://apple.stackexchange.com/questions/214348/how-to-prevent-mac-from-changing-the-order-of-desktops-spaces/214349#214349

System Prefs > Mission Control
Uncheck Automatically rearrange Spaces based on most recent use.

This will fix the order of all your regular Spaces - but not Fullscreen spaces, which always go to the right of existing numbered Spaces.

Note this cannot fix the Mac confusing which external screen is which. That's not user-controlled at all, & seems to occur mainly [though not always] when the external screens are identical.