ExtJS 4 menu position

I'm new to ExtJS so please don't throw a thunderstorm of downvotes right away. I've looked in documentation and tried few possible solutions, but nothing works. I need to change the default behavior of menus. I have a button that triggers a menu dropdown and it works all fine until the browser window is too small to show entire dropdown, in which case it 'undocks' and ExtJS is trying to fit it in the window. I need it to stay where it is, relative to the button that triggered it. How can I do this in ExtJS? Thanks.