In Firefox or chromium

1. visit http://urlshortener.wmflabs.org/wiki/Special:UrlShortener
2. enter the unsupported URL https://www.mediawiki.org/wiki/API:Web_APIs_hub
3. blank that out and enter the supported URL https://en.wikipedia.org/wiki/Sheep#Vocalisations

Result: upon entering the mediawiki.org URL the input box border goes red and the grayed-out right-aligned error label "URLs to domain www.mediawiki.org are not allowed to be shortened" appears (good). When I replace it with a good URL the input box goes blue and the error label goes away (good), but the input field remains very narrow, because it still has padding-right: 534px;

Expected: the input field goes back to being full width once there's no error.
Perhaps the error label should be reported outside the input field, in a regular errorbox underneath. The error text squishes a bad URL (what happens in a language where it fills the input box? and the gray looks disabled.

Workaround: enter "asdf". The right-aligned label becomes "Not a valid URL" and the input field is much less narrow, dropping to padding-right: 120px;