There's a lot of lingo around the wider web (which is itself lingo) so this is one of a series of short definition posts.
This is the display mode of the current web, like you'd see on any old desktop, laptop, tablet, or mobile phone running a web browser. You load the page and it just sits there, no matter how you wave it around or move your head.
I mean, really.
Flat display is one of the three display modes on the wider web:
Using Three.js to build a reusable border geometry for spatial UIs!
🌸 Updated potassium style system (KSS), now with margins!
🌸 The path to the vNext
🌸 A couple of new spatial controls
🌸 A brief intro to the existing samples
🌸 Building UI components that work in flat, portal, and immersive display modes