This is an intermittent bug.

When typing in input boxes I find that performance can be inconsistent. Sometimes, the cursor will jump to the end of the string inside the box, so I end up appending after the unit (e.g. "406 px+7"), which then errors.

Another example is the input box just not inserting characters at all. It seems especially prone to missing arithmetic characters such as + or - or /. For example, I meant to type "300+100 px" but instead it ignores the plus and displays "300100 px", which then error because it's too big.

I've attached a gif of some of the behaviour, but it's hard to catch the error when I'm recording the screen, so couldn't display the missing character case.