After installing 8.5.3 and a new release of Extension Library I started to play with the Mobile controls included in it. It was originally a separate project on OpenNTF but now the controls are part of library itself. So you need 8.5.3 server (with extlib installed on it) to run apps using these controls. Thanks to new updateSite.ntf included with 8.5.3 it is now far more easier to deploy ExtLib to your server.
After upgrading Notes Client on Linux to 8.5.3 (it is in fact related to update of embedded sametime to 8.5.1) I started to get an error message "Unable to initialize Computer, and it can't be used at this time. Processed unsuccessful response: null for: REGISTER..." and then there were some details about the connection. Quick Goggle search helped: How to suppress SIP registration failures in the Sametime 8.5x client
Just a quick addendum to previous post. MessageContext is available in 8.5.3 gold version too, although I did not find any documentation on it in Help, in Release Notes and in fix list database.
So this is probably "let's uncomment this and don't tell anyone" kind of fix :-)
But thanks a lot for it!