I noticed this on the API sandbox, but I can also reproduce on the demos, so this isn't API-sandbox-specific.

1. Go to https://en.wikipedia.org/wiki/Special:ApiSandbox#action=query
2. Switch to the query page.
3. Click the prop input, and select an item, e.g. categories from the menu.
4. Note that the menu closes (which is probably as expected).
5. Now try to get the menu open again.

Expected:
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.

Actual:
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.