Let's focus on filtering at the moment.

Stylesheets are sorted by their order in the document, which does have an impact on which rule might be applied. This is not really hinted anywhere in the UI, but changing that would require more work (maybe having a source tree, like in the debugger would be better).

The patch adds a filter input in the xhtml file, which will filter the list
of stylesheets displayed on the left side, based on the stylesheet full path.
If the active stylesheet is filtered-out, the first visible one gets selected.
If all the stylesheet are filtered-out, a label is displayed indicating that
the search doesn't match any stylesheet.

The splitview CSS already had rules for this behaviour, so we're reusing what's
already there, re-adding the placeholder for the "all filtered" message.

This had an impact on keyboard navigation; the list is sorted by ordinal property,
which drives a MozBoxOrdinalGroup style property (similar to order on a flex item).
The navigation used to simply get the active ordinal, and move to the next one.
Now, the "next" ordinal could be hidden, so we change the behavior to move the focus
to the next (or previous), visible item in the list.

A test is added to check that everything works as intended.

Depends on D144686

Backed out for causing devtools failures. CLOSED TREE

Backout link : https://hg.mozilla.org/integration/autoland/rev/162b06f558fba96669ee4b1671133a01203f315b

Push with failures : https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Crunnable&revision=9a28a5613dbd15e4ee5ccab74a2a0983dbbc24fe&selectedTaskRun=IzU_jJ5LTBixBoA6ZjoLaw.0

Link to failure log :
https://treeherder.mozilla.org/logviewer?job_id=376433724&repo=autoland&lineNumber=25619
https://treeherder.mozilla.org/logviewer?job_id=376432611&repo=autoland&lineNumber=3275
https://treeherder.mozilla.org/logviewer?job_id=376434105&repo=autoland&lineNumber=9430

I'm investigating the test failures

For some reason, the new test added in the previous patch has an impact on this test:
when checking the disk cache, there's a mochi.test:8888/tests.json entry, and
since the test was only checking if there was an entry with the mochi.test:8888 entry,
it was failing.
I wasn't able to understand why we have this entry in the cache, but it reveals
that the test check is a bit too broad, since what we cares about is simply to
know if the fetched stylesheet was added in the disk cache.
So here we're doing just that, checking that the fetched stylesheet wasn't added
to the disk cache.

Depends on D144723

https://hg.mozilla.org/mozilla-central/rev/c6d0be45b88c
https://hg.mozilla.org/mozilla-central/rev/1c7305c88a56

Nicolas, should we add this feature to our Nightly release notes? Thanks

(In reply to Pascal Chevrel:pascalc from comment #10)

Nicolas, should we add this feature to our Nightly release notes? Thanks

I think it's a feature that a lot of people using userChrome/userContent file have waited for and so it seems worth a mention in the release note.

(In reply to Pascal Chevrel:pascalc from comment #10)

Nicolas, should we add this feature to our Nightly release notes? Thanks

yes, I guess it's a nice addition that's worth mentioning :)

Note added to our Nightly release notes with this wording:

You can now filter style sheets in the Style Editor tab of our developer tools