I really like how, on older calculators, the comma (used as thousands-separator) moves as you input a large number. It makes it very easy to enter such numbers quickly and without errors.

This has been eliminated in newer calculators, including the HP-50g and the HP-35s.

I have heard repeated that new calculators cannot use this as they accept a more general form of input on the stack. But I think this explanation does not really hold up. It would be very easy to write an algorithm that places commas properly even for arbitrary stack input: simply check that to the left of the cursor you only have digits until a whitespace, and if so, place commas every three digits (taking into account of the decimal point). When a non-digit (such as E, or -, etc) is inserted, freeze the commas where they are, and continue getting input.

Why would this not work?

In fact, it would be quite simple to encode the above as a state-machine.

Luca