Consider a modal dialog where a user can select an item from a list.
Option 1: Instantly close the modal upon selecting an item from the list.
Option 2: Have a selected state for the selected item and provide OK / Cancel buttons to confirm the selection before closing the modal.
Which of the two options is preferred from an accessibility standpoint? (considering keyboard navigation support, etc). Note that the selection is non-destructive and the user could easily re-open the modal and change the selection.
Option 2 is your best option here is some guidance related to this and the WCAG guideline
Basically do not change context, update lists etc. automatically on any type of input, you should always give the user a chance to confirm their actions.
Now the question is do you really need a modal if all the user is doing is selecting an item from a list?
Surely it would be a better UX to just have a list you can choose from (this depends on what you are doing, just thought I would point it out as you didn't give an example).
You also need to consider the fact that your modal list should present number of options, currently selected options etc. etc. Yet again these are just guesses and something to consider, it could be irrelevant.