The AutoComplete widget has a different purpose than the ComboBox/DropDownList, that is why it expose different keyboard navigation behavior.
The AutoComplete is more a suggestion box (just like the Google search bar). The widget shows suggestions in its popup and the user could loop through them without selecting anything (the items are just focused and not selected). That is why the user could reach the last one using DOWN arrow, un-focus it and focus the first one. This is the intended "loop" functionality.
The ComboBox/DropDownList, in contrast, directly select the next/prev item. They behave just like the native HTML Select element. Because the item is directly selected, we avoid looping the suggestion list, because this will result in unexpected selection. This could not happen in AutoComplete/MultiSelect because the user needs to hit ENTER explicitly.
The AutoComplete is designed to open its suggestion box only when there is initiated filter. It doesn't have toggle button by purpose. Again this is intended.
To sum up, the described AutoComplete behavior is intended and we cannot provide any workaround to change its implementation. If you would like more ComboBox/DropDownList components, then I would suggest you use them instead. For instance, you can hide the ComboBox toggle button, resulting in AutoComplete look and feel, but preserving the desired ComboBox behavior.
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