We are seeing this issue here too, but only on one of 5 PCs. We tried deactivating all extensions but that doesn't seem to affect the issue. At the affected PC this also is reproducable with Chrome Canary.
Seems the key to reproduce this bug is activating the Chrome Flag:
... which you can access by typing into the URL: chrome://flags
That was the only difference to machines where I saw comboboxes working
in the exact same (latest) Chrome version.
So again the question: will ExtJS3.4 Comboboxes stop to work as soon
as EcmaScript 6 will be the official standard built in into all major browsers?