Summary
When a user posts a LINK with either http or https to a any site external to the domain on which Mattermost is served sees the link opening in a 'new' browser session with some text on the top left of the screen offering a 'Return to Mattermost'. Opening links on the same DOMAIN as the Mattermost instance resides on open in a closed window; with no option to return to Mattermost. The only way to return to the team is to close the application completely and log back in.
Steps to reproduce
Open a URL which is external to the Matterhost domain/host - you see the 'return to mattermost' link top left
Open a URL which is internal to the Matterhost domain/host - you see the site raw with no option to return to 'mattermost'
Expected behavior
I would expect that the option to 'return to Mattermost' is consistent for any URL's be they 'local' or 'remote'
Observed behavior
As described above. This behaviour has been reproduced by multiple users on our Mattermost instance.