I am having problems in ExtJS 4 with DD (it happens in Chrome, IE). I initially noticed it in Grid because of DD actions. Basically, this is what happens:
Viewport is loaded in maximized browser window
User restores window to smaller size
If user does any kind of drag gestures with mouse going out of the bounds, whole viewport "scrolls" away
Now it is happening in application I am currently working on, but I also reproduced it in blank sencha fiddle window. Here is the screencast so you can better imagine what happens: http://www.screenr.com/61AH
Try to open sencha fiddle (https://fiddle.sencha.com/) in maximized Chrome window, close sidebar (like in screencast), restore Chrome window to smaller size and than click and hold anywhere in window + move cursor out of the window at bottom. Whole viewport should scroll up.
Now reproduction scenario in application I am working on is a little bit different, but symptoms are the same. I cant post screencast of that application so I did it again in sencha fiddle a little bit slower: http://www.screenr.com/HdKH