How to use Scroller/Draggable without any boundaries (window)?

My problem is as follows: I have a Panel that has <img> tag inside that fills entire space. That all together makes a 'viewer' that is capable of showing pictures with server side zoom/pan etc. To 'simulate' the pan action on client side I need to move img tag inside the panel. I have tried to use "drag" event to set offset, but it messed up entire UI.

I have tried using Draggable, but I haven't found a way to drag it outside boundary of parent panel.
Than I have discovered Ext.util.Scroller, that seams to work ok for my needs, but the image should NOT come back to original position, and should not come back into parent panel boundary. I can't force the second condition.

With this code it will not come back, if it fits inside parent panel, but will come back if it gets outside.

new Ext.util.Scroller('viewerImage', {
revert: false,
bounces: false,
momentum: false,
springTension: 0

I hoped to use "constrain" option set to null, but than I get
Object [object DOMWindow] has no method 'addCls'

Any idea how to make that type of draggable/zoom'able background image?

I have found in Sencha code, that I need to disable "this.snapToBoundary();". It would be nice if that would be a configuration option to be able to scroll objects out of view. This is normally not needed, but useful in some applications.