Support matrix URI scheme

[turt2live]

matrix-org/matrix-spec-proposals#2312

The MSC has been accepted and will be in an upcoming version of the spec.

[KitsuneRal]

I guess adoption of Matrix URIs will look like follows:

1. Element (and other) clients catch up with recognising Matrix URIs and converting them to matrix.to links. Conversion from matrix.to to matrix: is also implemented but is off by default.
2. Once sufficient portion of the client ecosystem does step 1, the switch in clients flips to translation of matrix.to URLs to Matrix URIs on the wire, while still recoding them back to matrix.to links for displaying (so that integration with web browsers is not broken, e.g.).
3. Independently from above, matrix: becomes recognised as a safe URI scheme by web browsers at some point, so that web clients could register themselves as handlers for matrix: links.
4. Widespread adoption; clients start using Matrix URIs for hyperlinks in the UI and stop recoding between matrix.to and Matrix URIs in general cases. The ability to generate matrix.to links when requested by the user is retained for back-compatibility. At this point I expect matrix.to and matrix: links to co-exist on (almost) equal foot, with the default URL form gradually turning to matrix:.

5. If/when web browsers become able to natively deal with matrix: URIs, they will automatically direct either to the client of choice (if already registered - as implemented in step 3), or show some basic data on the Matrix resource pointed to by the URI along with links to full-blown clients, web or not (effectively incorporating a matrix.to service page). That would mean implementing a very small extension for browsers with the sole purpose of retrieving a Matrix resource by the URI or just rewriting the Matrix URI as a matrix.to URL.

[turt2live]

From TWIM, some useful links:
https://bugzilla.mozilla.org/show_bug.cgi?id=1688030
https://bugs.chromium.org/p/chromium/issues/detail?id=1169258

[bwindels]

Also see matrix-org/matrix.to#191, including my thoughts on being able to switch to matrix: for generating share links.

[Mikaela]

Does this issue include Element on all platforms or are there separate issues to follow? I have started using matrix: URIs to not link to matrix.to.

[aaronraimist]

@Mikaela this issue is only for Element Web and Desktop.

element-hq/element-ios#4297
element-hq/element-android#3268

[Mikaela]

Are this (and #18655) planned in near future? I have been using both more or less in production since August and I understand them to be part of the spec (at least #18655 is) and I kind of have to tell people to not use the Matrix flagship client, Element, as it's missing these features of Matrix with the unsupported links being significant and more often encountered problem (due to matrix-org/matrix-spec-proposals#3386).

If Matrix URI support is not going to happen in Element, will they at least be converted to matrix.to URLs internally alongside matrix-org/matrix.to#191 ?

[KitsuneRal]

I hate to say that but with the spec PR merged and quite a few (also Web-based) clients supporting the URI scheme Element (across platforms) becomes a laggard holding the scheme from wider adoption. I'm not even talking about full-blown support as yet but at the very least the top bullet from the list I laid out in the first comment is not contentious at all, is it?