The classic "fun" TSR use-case was to make an app that installs an interrupt vector to decode mouse hardware "events" directly from the RS232 port before exiting via 0x21h, so that DOS screen would display a pointless "native" mouse cursor that doesn't do anything :)
When I say mouse cursor, I mean an ASCII block character with the blink bit on.
Some mouse drivers used an upper ascii range (less used) and rendered 4 “arrow” cursor parts into it over 4 chars over which the cursor ought to be. And then replaced those 4 chars on screen temporarily. As a result you had a fully pixel-perfect cursor in text mode.
You're right, TUIs had mouse support. This is a TSR, and exits to shell after running, so there's a mouse cursor visible without any app running. Like, there's nothing to "click" :)
The classic "fun" TSR use-case was to make an app that installs an interrupt vector to decode mouse hardware "events" directly from the RS232 port before exiting via 0x21h, so that DOS screen would display a pointless "native" mouse cursor that doesn't do anything :)
When I say mouse cursor, I mean an ASCII block character with the blink bit on.