When you are in-game, and press the "Start/Stop Steam" button while not having a linked Mojang account will cause a window to pop up about link the accounts.
But when you press the "Cancel" button, nothing happens until you press it again, since it seems like it created 2 overlays of the Link account overlay.

And if you press the "Start/Stop Stream" button again, while still in the Link account overlay, a new overlay will appear, though you won't be able to see it, you will need to press an extra time on the "Cancel" button to get out.