Mac os Sidecar is just it’s built-in dual or multi monitor mode.
This is incredibly odd, but if you move Sublime Editor into a secondary window ( ie. a dual monitor ), it’s view state is frozen. If you drag it back, you can see it is editable. In other words, once I drag it into a different monitor, none of my editing or even the cursor is blinking. Drag it back, and everything works as I assume it would ( including any blind changes I made to it while in the sidecar ).
Very strange indeed because Sublime is the only application that behaves like this. And I can not for the life of me understand why that would be the case.