Success! Looks like we've fixed this one. According to our records the fix was applied for
Ext JS Premium Member
What is the status of EXTJSIV-6520? This is causing problems with our test tool from HP. It seems the strange 20000px item that has been added in 4.1 is causing this. Can Sencha use another technique rather than this to overcome whatever shortcoming they were trying to fix?
This is still open, but slate for 4.1.2 (no release date).