It’s not the same thing but I’d like to add that the standard user experience on mobile when using any dropdown/select/modal is the Back button to close something that’s open:
At least for me it’s pretty common to be on mobile editing a tag or category for a topic and intuitively hit Back to close the dropdown and end up in the previous page. I’d love to see all modals and dropdowns on Discourse behaving the same as native on mobile, but I agree that a more explicit way of closing dropdowns will be a step forward to avoid this confusion.