`AutoCompleBox` drop-down cannot be reliably opened

Describe the bug Opening the drop-down of AutoCompleteBox is unreliable and mysterious, for both users and control consumers. Behaviour is particularly confusing when MinimumPrefixLength is 0.

To Reproduce

1. Open the Avalonia Control Gallery and focus on an AutoCompleteBox with some items defined.
2. Press the down arrow key
3. Nothing happens
4. Type a character that matches an item
5. Cancel the pop-up
6. Press the down arrow key
7. The drop-down opens again
8. Clear the box and type a character or string which doesn’t match any items
9. Press the down arrow key
10. Nothing happens.

Expected behavior The autocomplete drop-down should open whenever the down key is pressed, or at least some feedback should be given as to why it did not open.

This bug also means that the drop-down cannot be opened from the outside unless certain pre-conditions that the control decides on are met. I expect setting the IsDropDownOpen to always open the drop-down.

Desktop (please complete the following information):

• OS: Windows
• Version 11 preview 1

[Philltorian]

I ran into the same issue a while back, I managed to get the desired effect (arrow down and F4 opens the dropdown when blank) by using the attached behaviour.

AutoCompleteZeroMinimumPrefixLengthDropdownBehaviour.cs.txt

[AtomCrafty]

Yes, already on it 👍