The idea is:
You can convert a field to be an expression by starting with an “=”, I love the feature but the field loses focus when entering “=”. Making it not faster than just clicking “expression” to convert it.
Would love for it to keep focus on the field so we can just click the field and keep typing. 
Are you willing to work on this?
Is probably something that I am unable to do easily as it is Front-end stuff.