Test case.
Go to any page with extjs. ExtJS API Documentation for v3 is fine
Open firebug console.
Type
PHP Code:
w =  new Ext.Window({width:1500,layout:'fit',height:100,items:new Ext.ComponentMgr.create({xtype:'buttongroup'})});w.show();w.setWidth(1550) 
Stare at two vertical lines. They are here because a background image for them is
http://dev.sencha.com/deploy/ext-3.4...n/group-lr.gif which has width 1000px. Who would think that a button group can exceed 1000px width, right?

So a workaround: open gimp,open the gif, scale the image to 10000 x 1 size, make sure 2 first and 2 last pixels are correct while remaining are transparent, then save a gif