IE9 is a big step up, and any errors in the IE9/ExtJs combo is entirely ExtJs fault as it uses faulty UA sniffing to adapt.
There are several areas where ExtJs will actually think that IE9 is < IE6 due to the browser not being IE6 || IE7 || IE8. This is for instance the case with drag and drop.
Again, this entirely ExtJs' fault - it could easily have done feature testing on most (or all) of these.
You have got to be kidding me... Do you realize that Microsoft routinely introduces new browers which support new features or forgo old features many of which ignore standards and use proprietary implementations. Do you have any idea how much easeir the universe would be if IE would just follow standards?
You can blame who you want, but it isn't "ExtJs' fault". It's Microsoft for refusing to comply with open standards. And that's ignoring the fact that MS only recently killed support for a 9 year old browser version. There are plenty of things that ExtJs could do better (CompositeFields being a container, RowEditor actually working etc...). But at the end of the day the craptastic browser that is IE is due to Microsoft's unwillingness to use open standards and (perhaps more importantly) the fragmentation they cause by refusing to end-of-life products that (by web standards) are ancient.