Should be pretty easy to do now that there is CompositeControl.
The control should have two states, dropdown shown or not.
Without the dropdown shown, it looks like a button but with a chevron of some sort.
The mouse bounds for non-dropdown mode would be the single row.
If the chevron is pressed, the control requests exclusive focus and draws the rest of the control, showing the listbox. Mouse bounds would change to be the full area of the control
Should be pretty easy to do now that there is
CompositeControl
.