Currently in version 16.0.138 of Keyman for Android, long-pressing a particular key shows key(s) as pop-ups to select one.
After focusing the pop-up key using touch, releasing the touch will trigger that key.
If we can make focus to the first key among the pop-up keys set by simply long-pressing only, it will be useful to users to type using keys that have one option under long-press.
Kind attention is invited to this matter to add to a future milestone.