You mention "When the default OOTB autocomplete control is shown, if you type in the input field and press the down keyboard button, the full list is shown."
I want to clarify this statement. If you set focus on the autocomplete field by putting your mouse/keyboard pointer and press the down arrow key, you would be able to see all the values for the autocomplete. If you start typing in a value in the field, even without pressing down arrow key, the filtered values start appearing. You can try this in the UI gallery sample.
If your question is specifically about minimizing a key stroke so that the moment focus is set on the autocomplete field, you want all the values to be shown, there is no different OOTB control to do it at the moment.
Recently our business team also wanted a similar functionality, ie, for the auto complete control to behave as a dropdown too. When it is clicked inside the control, we needed the options to be opened automatically rather than while typing inside or pressing the up/down arrow. I had opened a product enhancement SR but was redirected here. I just wanted to know whether there is any consideration for the implementation of a similar functionality anywhere in the near future.
thank you for your response. Yes, my scenario is related to the second part. Currently the screen in question can be done by tabbing and selecting or just my selecting via mouse clicks (except OOTB autocomplete). A user more adept at using the mouse would need to switch from mouse to keyboard(down button) and then select via mouse and then the rest of the items (again via mouse). It overall makes the UX slightly cumbersome for the user.
I agree about the challenge/concern regarding the custom control, but a UX vs a requirement is a requirement vs maintainability is never an easy decision... :(