[w3c/manifest] Add Optional Opt-In for UA Default Transition (#504)

This issue proposes adding an optional opt-in for a web app to request a UA show a default transition.  Something like:

`"transition_fallback": "platform_default";`

Many web apps will either not need transitions or will write their own code to handle transitions.

However, some web apps (which may start from progressively enhancing existing web pages) may not have code to handle transitions and may want some native-like fallback transition UX.  

FAQ
**Q: Who is this for?**
A: Web app authors who may want to do minimal initial work to convert a legacy website into a web app but still get as much of a native-like experience as possible.

**Q: What might a "platform_default" transition look like?**
A: If a page render had not completed after a user-perceptible amount of time, (ex: 1 second), the UA might show a transparent overlay and some UX widget like a spinner, loading bar, hourglass, watch, etc.

**Q: Do all UAs need to support having a default transition?**
A: No, many UAs may not do anything (and web app authors should expect this.)

**Q: Will this break any existing web apps (and web app manifests)?**
A: No, this is an optional, opt-in feature.

**Q: Will this require any additional work from web app (and web app manifest) authors?**
A: No, this is an optional, opt-in feature.

**Q: Does link rel="prerender" solve this?**
A: Prerender works great if the navigation is linear, but in a number of cases, navigation is non-linear:

- A game in which a user can visit one of multiple "neighbors"
- A game in which a user can choose to play one of multiple unlocked levels
- A social experience in which a user can navigate to one of multiple "friends"
- A financial app in which a user can check the performance of one of multiple stocks/funds/etc.

**Q: Would any additional values beside "platform_default" be allowed?**
A: No, not for now, this could be expanded in the future, but the intent is that this gives web app authors a way to request a platform default transition until they have time / resources to write their own transition code.  

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/504