Even simple combo boxes do not work: https://fiddle.sencha.com/#fiddle/p5, we've traced it down to something about the way loops are handled in XTemplates. It appears to be working fine on 4.1 and later with the introduction of XTemplateCompiler. It also works okay on Chrome 29 and lower.
...hmmm, it looks like it might be specific to my machine; I've asked a few colleagues and they are able to run it just fine. Please close the thread and I'll file a new bug if I can get to something more specific