I have a lot of problems when ExtJS (v4.0.7) does not identify IE11.
In this case occurs many UI bugs under IE11 browser.
I couldn't use "flax: 1" for grid columns and other layout stuff.
I found the solution which brilliantly fixed all my problems under IE11.
So, I have the HTML where I include next files:
In lib-overrides.js I insert the code which fix IE11:
Ext.apply(Ext, ((a = navigator.userAgent) && /Trident/.test(a) && /rv:11/.test(a)) ? {isIE:true, isIE11:true, ieVersion: 11} : {});