[w3ctag/design-reviews] Web App Launch Hander (Issue #683)

Braw mornin' TAG!

I'm requesting a TAG review of [Web App Launch Handler](https://github.com/WICG/sw-launch/blob/main/launch_handler.md).

Web app launch handler is new `launch_handler` manifest member that enables web apps to customise their launch behaviour across all types of app launch triggers.
We found that almost all "launch" use cases could be covered by a select few fixed rules (for example, "choose an existing window in the same app, focus it, and navigate it to the launched URL"). This `launch_handler` proposal enables sites to specify a set of fixed rules without having to implement custom [service worker `launch`](https://github.com/WICG/sw-launch/blob/main/explainer.md) event logic, which should satisfy most use cases, and simplify the implementation in browsers and sites.

This is a successor API to [Declarative Link Capturing](https://github.com/WICG/sw-launch/blob/master/declarative_link_capturing.md) which has its own completed [TAG review](https://github.com/w3ctag/design-reviews/issues/589).

  - Explainer: https://github.com/WICG/sw-launch/blob/main/launch_handler.md

  - User research: Positive developer feedback from the [DLC origin trial](https://developer.chrome.com/origintrials/#/view_trial/4285175045443026945) wanting to continue using the `navigate-existing-client` behaviour.
  - Security and Privacy self-review: [Web App Launch Handler: Security and Privacy self-review](https://docs.google.com/document/d/13DmlypZBVbEGiwmOJLV1E4PVmm2_6wh9feBgit5wW6g/edit?usp=sharing)
  - GitHub repo: https://github.com/WICG/sw-launch/issues

  - Primary contacts (and their relationship to the specification):
     - Alan Cutter @alancutter, Google
     - Matt Giuca @mgiuca, Google
  - Organization/project driving the design: Google
  - External status/issue trackers for this feature:
    - https://www.chromestatus.com/guide/edit/5722383233056768

    - https://bugs.chromium.org/p/chromium/issues/detail?id=1231886


Further details:

  - [ X ] I have reviewed the TAG's [Web Platform Design Principles](https://www.w3.org/TR/design-principles/)
  - The group where the incubation/design work on this is being done (or is intended to be done in the future): WICG
  - The group where standardization of this work is intended to be done ("unknown" if not known): Web Apps WICG
  - Existing major pieces of multi-stakeholder review or discussion of this design: [Declarative Link Capturing TAG review](https://github.com/w3ctag/design-reviews/issues/589)
  - Major unresolved issues with or opposition to this design:
  - This work is being funded by: Google

You should also know that...

`launch_handler` is a piece of the larger web app link handling puzzle:
[PWA Link Capturing and URL Handling: The New Taxonomy](https://docs.google.com/document/d/1w9qHqVJmZfO07kbiRMd9lDQMW15DeK5o-p-rZyL7twk/edit#)

We'd prefer the TAG provide feedback as:

  💬 leave review feedback as a **comment in this issue** and @-notify [github usernames]


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