The "Share Desktop" button always shows up, although it should only be displayed if this contact supports Jingle. If the other contact is not running such a client it will always fail. Jingle is also needed for calling and this call button is shown appropriately. So showing the "Share Desktop" button **without** the call button should be a contradiction. I mentioned the bug [here](http://lists.jitsi.org/pipermail/users/2015-October/010175.html).
I suspect the bug could be [here](https://github.com/jitsi/jitsi/blob/master/src/net/java/sip/communicator/impl/protocol/jabber/OperationSetContactCapabilitiesJabberImpl.java#L242) or rather in those classes [here](https://github.com/jitsi/jitsi/blob/master/src/net/java/sip/communicator/impl/protocol/jabber/OperationSetDesktopStreamingJabberImpl.java), [here](https://github.com/jitsi/jitsi/blob/master/src/net/java/sip/communicator/impl/protocol/jabber/OperationSetDesktopSharingServerJabberImpl.java) or maybe [here](https://github.com/jitsi/jitsi/blob/master/src/net/java/sip/communicator/impl/protocol/jabber/OperationSetDesktopSharingClientJabberImpl.java).
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Reply to this email directly or view it on GitHub:
https://github.com/jitsi/jitsi/issues/186