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.