gistfile1.js

//Override
Ext.override(Ext.Button, {
    preventFocus : false,
    // @override
    onMouseDown : function(e){
        if(!this.disabled && e.button === 0){
            if (this.preventFocus) { // injected line here
                e.stopEvent();
            }
            this.getClickEl(e).addClass('x-btn-click');
            this.doc.on('mouseup', this.onMouseUp, this);
        }
    },
    // @override
    onMouseUp : function(e){
        if(e.button === 0){
            if (this.preventFocus) { // injected line here
                e.stopEvent();
            }
            this.getClickEl(e, true).removeClass('x-btn-click');
            this.doc.un('mouseup', this.onMouseUp, this);
        }
    }
});

//Usage

new Ext.Button({
    preventFocus : true,
    text         : 'I do not focus on click!'
});