Don't expire sessions
There is no technical reason to expire sessions whereas currently Excel Online (and word) get their sessions expired after a few hours. Please could the session model be fixed so that sessions expire only when deemed necessary for security reasons (at which point a re-login would be required) i.e. just like google docs.
Automatically refreshing the page would be a short-term workaround.
This problem prevents online applications having parity with desktop applications, even where the required feature subset is fully supported.