OO.ui.NumberInputWidget only accept numbers as input, that are multiples of the step value (or actually the diff to the minimum must be a multiple of the step value). This is consistent with native number inputs (https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/number), but I think it is unexpected and probably unintentional:
- It's not documented.
- There is an allowInteger option that defaults to false, but as the default step is 1, only integers are allowed anyway.
- The example https://doc.wikimedia.org/oojs-ui/master/demos/?page=widgets&theme=wikimediaui&direction=ltr&platform=desktop#NumberInputWidget-0-1-step-by-1-page-by-25 has step 0.1, but pageStep 0.25. So using "Page up" once will make the input invalid. (Actually, due to rounding errors, even with the simple step you'll end up with an invalid value sooner or later, T102128: Decimal step on NumberInputWidget results in cumulative binary rounding error)
- The tooltip that tells me about the invalid input is in German, even though the rest of the page is English.
So I think this behaviour hasn't been the case from beginning. Either revert to make NumberInputWidgets accept any number no matter what the step is, or document the behaviour properly and make sure the default options and examples make sense.