(This answer relies on a 4-dimensional being, capable of "seeing" the past/future, being able to 'look around' 3D objects in the same way that we do 2D lines - even if they don't quite realise that that's what they are doing)
Start with 2 layers a special material (Such as e-ink or a liquid crystal) which can change via some mechanism, such as heat or electricity.
The top layer (Layer α) can switch, almost instantly, between Opaque and Transparent.
The bottom layer (Layer ß) can switch, almost instantly, between "Message" and "No Message".
When the Message is Visible on Layer ß, it is completely obscured by Layer α being Opaque. When Layer α is Transparent Layer ß is blank. Because the layers are so tightly fused together, it is impossible to change one without simultaneously changing the other.
However, by focusing on the future (or, alternatively, on the past) of Layer ß, while Layer α is still transparent, you are able to see the message that it will show when it is obscured.
