GWT submits a FormPanel into an iframe and then waits for the iframe to fire a load event in order to fire a SubmitCompleteEvent. Looks like Firefox and Safari do not fire a load event on the iframe in case loading fails with status code 400. I am unsure if Chrome or Firefox / Safari behave correctly.--
The only workaround I could think of is using XMLHttpRequest and send the data to the server yourself (and thus gain access to the http status code) : https://developer.mozilla.org/en-US/docs/Web/API/FormData/Using_FormData_Objects Browser support is pretty good: https://caniuse.com/#search=formdata
GWTs XMLHttpRequest does not have an API for it built-in but with JsInterop / JSNI or Elemental you can easily make it work.
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