Hi Denis,
I understand your confusion. The idea is that the "clip to window" setting should be forced ON for touch devices since scroll bars are usually not very user friendly in our case. However I see now that this has been lost somewhere along the way. I have now pushed a fix to noVNC master. You will need to clear your web storage for your noVNC page in order for the fix to have an effect (the old setting is saved in web storage).
The conditions for view drag are:
1. "clip to window" enabled
2. remote desktop size is larger than browser viewport