[Web Install] Prevent installation if there is an existing installed app that controls the same scope

[amandabaker]

Originally mentioned here by @dmurph: #893 (comment)

I think we should consider preventing install if there is an app already installed that controls the start_url of the app, or the current document in the same-origin case. This is mostly to prevent this kind of use, which hinders the UA's ability to promote installation / know which urls belong to which app (and other problems cited in here. Then, when we solve w3c/manifest#996, devs can make sure the places they need to installed (if nested) fall outside of the outer app's scope.

I think this sounds like the right approach, and here are some initial pros/cons:

• Pros:
  • Less likely to cause confusion around which app should open the content.
  • Aligns with the current install prompting behavior in chromium-based browsers
• Cons:
  • Some devs want to support nested apps and don't have a way to do that until Scope: Allow developer to have more fine-grained control of app scope. w3c/manifest#996 is addressed.
  • You can get into a state where two apps control the same path if an app with a smaller scope is installed first (e.g. example.com/app/cool-game) then a larger scope second (e.g. example.com/app).
  • Existing "Site as app"/"DIY apps" installed by the user could prevent nested apps from being installed (this issue already exists for browser-prompted installs)

[dmurph]

I would be OK with us saying that those manual apps don't prevent installing of nested stuff / are 'last on the list' when it comes to considering who 'controls' a url. That.... may make things even more complicated lol. But if we end up having problems here with manual apps blocking install, we could explicitly exclude those.