I wrote a big comment next to the changes I made.
This is a redo of #17961 which had various issues.
Closes#17916Closes#18070
## Validation Steps Performed
* Pressing Enter within the input line doesn't crash ✅
* Type "Cour" and pick Courier New, press Save = Saved ✅
* Pick any other font from the dropdown, press Save = Saved ✅
* Picking an option dismisses focus but not to the tab row ✅
* The first time after launching the SUI, when the setting is still
unmodified, when you focus the box and pick an option,
it'll unfocus the box ✅
* When the setting is unmodified, and you pick the default
(Cascadia Mono), it'll still unfocus the box ✅
8d3f12b1c0