I solved this but posting it in case someone else finds it useful.
Aim: have an input semi-linked to a dropdown. When the dropdown changes, change the input value. But as soon as you change the input manually, lose the link to the dropdown value.
Result: this is really easy to do which I found surprising. Just set the “initial” value of the input to follow the dropdown value.
So the “Initial” value of an input is not just the initial value - it is the default value unless and until you change the input yourself manually.