If the session is larger than the browser window there is no way to pan or scroll in 10.10 and newer versions of OS X.
How to reproduce:
1. Open a session using the HTML5 client in Safari on Mac OS X 10.10 or newer
2. Make the remote session larger than your browser window using, for example, xrandr
3. Parts of the session is now outside of the viewport and you can't scroll or pan
The user can in the settings for Safari disable auto-hide scrollbars to work around this.
Fixed upstream by forcing clipping mode in Safari:
Fixed in upstream noVNC now.
Verified that ThinLinc 4.7.0 made Safari to show pan tool in toolbar instead of scrollbars. Tested against tl-4.7.0post_5377.r32212.jenkins543 that not pan toolbar is shown and the scrollbars works as expected.