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Chromium uses the same internal rendering engine etc. as Chrome, from source code released by Google. Effectively Chromium is Chrome minus a few Google proprietary or separately licensed parts such as the built-in PDF viewer.
Vaadin is not automatically tested on Chromium, but should work just as it does on the corresponding version of Chrome.
Thank you for your answer. The reason I was asking is because I'm seeing some problems with Vaadin applications on that browser and I didn't want to bother developers with Trac tickets if it's irrelevant. So I guess it does make sense to report then?