This is pretty minor and fairly inconsequential, I think it’s a bug though so just posting it here. This is on Stable 2.7.1.
So when your open the editor for a theme component (i.e. to edit CSS) there is a save and preview button. Once any change has been made, the preview button disappears and only save can be used.
I think this is a bug, as I believe the preview link is normally for seeing the changes prior to saving them.
Edit: Unless I’ve misunderstood and the purpose is to preview components that aren’t implemented on a live theme. If I delete changes (reverting back to exact match the previously saved version) the preview button remains hidden.