gistfile1.js

// *NOTE* : This is currently certified for Ext JS 3.0 - 3.3.1
var win = new Ext.Window({
    height : 200,
    width  : 300,
    layout : 'fit',
    items  : {
        xtype : "textarea",
        value : 'Click here, then click a button!'
    },
    buttons : [
        {
            text : 'Steals focus!'
        },
        {
            text : 'Does not steal focus!',
            // @override
            onMouseDown : function(e){
                if(!this.disabled && e.button === 0){
                    e.stopEvent(); // injected line here
                    this.getClickEl(e).addClass('x-btn-click');
                    this.doc.on('mouseup', this.onMouseUp, this);
                }
            },
            // @override
            onMouseUp : function(e){
                if(e.button === 0){
                    e.stopEvent(); // injected line here
                    this.getClickEl(e, true).removeClass('x-btn-click');
                    this.doc.un('mouseup', this.onMouseUp, this);
                }
            }
        }
    ]
});
win.show();

win.items.items[0].el.focus()