Fix well-known lookup for sliding sync labs check

[t3chguy]

No description provided.

[richvdh]

How was it broken?
When was it broken?
Shouldn't we have a test for this?

But ok I guess

[t3chguy]

Introduced by #12307

[EdGeraghty]

Introduced by #12307

What was, exactly?

[t3chguy]

@EdGeraghty the code which caused the bug this fixes

[EdGeraghty]

@t3chguy that's what I'm asking. Which bug?

If it's respecting paths in the proxyurl, that's not a "bug" I introduced, it's the behaviour of the original check and the current behaviour in ElementX...

[t3chguy]

It was checking the base url not the server name domain. The well known is on the server name not the base url - see https://spec.matrix.org/v1.10/client-server-api/#well-known-uri .

The url constructor check is unrelated to the fix and the msc is unclear whether the path is absolute or relative, I'd argue the former given the specified path starts with a slash. This fix was to un-break the issue when a trailing slash was present on the proxy url as was exercised in the UT https://github.com/matrix-org/matrix-react-sdk/pull/12519/files#diff-7b41a4a2b051bc32d9905662ef8910daed47905334eb2dadb3b0d19be3142ec2R265

it's the behaviour of the original check and the current behaviour in ElementX...

Just because its how EX works doesn't make it right, what's documented in the spec or in this case MSC is what is right. The MSC currently outlines an absolute URL path, not a relative one.