What browser are you using? The input type="number" has different effects on different browsers.
For example, I’m using Safari. If I type a comma in manually, the stepper doesn’t recognize the field as a number anymore and will start it at 0 again. However, when testing on Firefox it recognizes the number and takes out the comma.
I believe in a type number with locale US, comma is not considered a valid character. You should instead consider entry with only 1234567890. and format the number with the thousand separators in lostfocus.
[quote=295682:@Hal Gumbert]Interestingly, I can type 1,000 and exit the field.
But NumbertTypeTextField.Text = “1,000” results in nothing in the field.[/quote]
To see how the browser handles that value, try to change it with the stepper.
As noted, Safari throws your value out, Firefox removes the comma, and Chrome is not welcome on my machine.