From what I've read in other posts ExtJS doesn't support compability mode.
What I've done myself to get around it is on pages with ExtJS code I include the following head tag:
<meta http-equiv="X-UA-Compatible" content="IE=8" />
Our users have IE8, so that works.
From what I've read you can instead use:
<meta http-equiv="X-UA-Compatible" content="IE=edge">