Bug 1576624 introduced the ResourceWatcher API, accessible via toolbox.resourceWatcher. This API will help listen to data that is being created early, when the document just started loading.

We should migrate the whole DevTools codebase to this API for any data that:

• DevTools frontend listen to, or care about,
  and,
• may be created or be notified early, when the document just starts being loaded.
  This data will typically be: console messages, errors, warnings, sources, Root element NodeFront, storage values, network events, stylesheets, ...

We are typically not going to use this API for:

• data being fetched on-demand, from user's input. For ex: webconsole evaluation input, or, DOM element expands from the Markup view.
• events which we only want to record when the user cares about them. For ex: animation events.

For some more high level context, please have a look at Migration to Fission-compatible APIs, which describes all Fission-related refactorings.

The typical task for this bug will be about migrating code that:

• start listening and register a RDP event listener,
• retrieve already existings data,
  from panel's codebase, to the ResourceWatcher module, in the LegacyListener object.
  And then, the panel should use the ResourceWatcher instead.

Bug 1620234 is a good example of such migration, applied to Console Messages.
Bug 1623699 is also useful example as it demonstrates how to write tests for such migration.

This bug is about focusing on only one usecase: the CSS changes.

We do request already existing changes, and start listening for the next ones from here:
https://searchfox.org/mozilla-central/rev/9c6e7500c0015a2c60be7b1b888261d95095ce27/devtools/client/inspector/compatibility/CompatibilityView.js#157-165

Tracking Fission DevTools bugs for Fission Nightly (M6) milestone

Depends on D74599

Depends on D74600

Depends on D74601

Tracking dt-fission-m2-mvp bugs for Fission Nightly milestone (M6c)

Backed out for perma failures on browser_inspector_sidebarstate.js.

Push with failure: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception&searchStr=windows%2C10%2Cx64%2Cdebug%2Cmochitests%2Ctest-windows10-64%2Fdebug-mochitest-devtools-chrome-e10s-2%2Cm%28dt2%29&revision=6d6323e559cb4fbc9d88ed2563230fc88f27f790&selectedTaskRun=WmYMfJqoT1SWE8hnV5cJiQ.0

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=306038173&repo=autoland&lineNumber=29510

Backout: https://hg.mozilla.org/integration/autoland/rev/593a07999f90a03ffda6dc4f620c851a11db5e7d

https://hg.mozilla.org/mozilla-central/rev/2c71a91762d3
https://hg.mozilla.org/mozilla-central/rev/dfd589598959
https://hg.mozilla.org/mozilla-central/rev/f3982a7f1771
https://hg.mozilla.org/mozilla-central/rev/e860655e21e7