RFC: Dropdown, selected item should be greyed out

[layershifter]

Extracted from #1795.

Active selection should greyed out to indicate that typing is allowed again.

Expected behaviour (with minCharacters)

SUI: Current behaviour (with minCharacters)

SUIR: Current behaviour (with minCharacters)

I believe that current behaviour isn't valid, however SUI has same problem.
@levithomason it will be awesome to hear your thoughts there.

[stale]

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