[Foldables] Support non-physical hinges

[kenchris]

In response to w3ctag/design-reviews#472

@torgo pointed out that though foldable phones, though they don't have a physical visible hinge (there is screen across the hinge), web apps still might want to consider the hinge area for animation etc

[zouhir]

@diekus and I met in person and I'd like to reiterate again what I said in our f2f and what my colleague @dlibby- said in his comment w3c/csswg-drafts#4736 (comment): this proposal was not meant to exclude foldable devices and we truly believe the ability for websites to snap to the hinge on a foldable device with logical display regions is as important as their ability to do so on a dual-screen device with 2 separate physical display regions.

it's just a matter for simple wording fix, I will do that.

[diekus-zz]

Indeed, i have an interesting chat with Zouhir. And Daniel Libby also stated the wording would be fixed. Just bringing this up since it hasn't been changed and thought it was useful to point this to the interested readers.

[darktears]

Something to consider as well is how a developer would be able to target both type of devices (folding and dual screens) yet provide a different UX.

Let me explain :

• On a dual screen device, provide a split screen UX where the top/left part of the app show a different content than the right/bottom part of the app.
• On a foldable device you can choose to provide a split UX or not. For e.g. early research shows that scrolling content on a foldable (half way fold) feels nicer and "magical" when the content flows across the fold, yet as a developer you still want to be able to position part of the UX in the right display region. For example if you're doing a scrolling photo gallery, on a half folded device you may want to display the gallery scrolling/flowing on both region, yet you need to know the regions to be able to position dialog or pieces of UX so they don't show up in the middle of the fold.

I'm not sure but maybe knowing the type of fold would be useful fold vs hinge for e.g. so we can let the developer decide what to do. On a hinge based device this is dead area, on a device that fold this is not, it can be used to draw things.