jquery/jquery-ui — pull request #1091 by tjvantoll (2013-09-27 17:53:21+00:00)
Title: Spinner: Only format the value when there is one. Fixes #9573 - Spinner:...

... forces a field validation in Firefox before field loses focus or form is submitted

There are already tests to ensure that non-empty values are formatted on creation (which actually saved me here), so I don't think any extra tests are necessary. Not exactly sure how we would test this anyways.