I noticed this on the API sandbox, but I can also reproduce on the demos, so this isn't API-sandbox-specific.
- Go to https://en.wikipedia.org/wiki/Special:ApiSandbox#action=query
- Switch to the query page.
- Click the prop input, and select an item, e.g. categories from the menu.
- Note that the menu closes (which is probably as expected).
- Now try to get the menu open again.
All (or at least some) of the following methods work, and will re-open the menu:
- Click the input again (while it is still focused)
- Press the cursor key for right, top, or down.
- Press the enter key.
None of these methods will open the menu, only the following methods work:
- Click somewhere to blur the input, and then focus it again.
- Press the cursor key for left, and then for right.
- Press the spacebar.