I have a modal window opening on button click, and it's height is about 800px, so it can be higher than browser window.
After modal window showing, it's content is cut under browser size, but browser window scrollbars do not appear.
Interesting thing is with mouse scroll (pushing mouse scroll button and dragging the mouse) we can scroll to hidden form part, although modal shade displays only on browser window size.
Tried to add autoScroll: true, autoHeight: true (excluding fixed height) to the window. Tried to render it to Ext.getBody() or Viewport.
The same behaviour appears when on window opening browser window size is big enough, and then it is diminished.
If the window is opened not by click, and displayed just on ready, browser scrollbars work correctly.