Steps to reproduce

1. Go to https://same-domain-iframe-stylesheet-reload.glitch.me/
2. Open DevTools and go to the StyleEditor
3. There should be 2 stylesheets, one for the top level document, the other for the same origin iframe.
4. Reload the page

Expected results

The list of stylesheet is cleared, and then the 2 stylesheet are loaded back

Actual results

The list of stylesheet is cleared, but only the top-level stylesheet is displayed
Stylesheets added through the "Add Stylesheet" button are not reflected in the StyleEditor either after the reload

I can reproduce on Firefox 90, regardless of Fission state.
I can't reproduce on Firefox ESR (78).

I did some investigation on this:

We rely on the document.styleSheetChangeEventsEnabled to be notified about stylesheets being added.
This flag is added by the StylesheeManager on all the documents of a given target when we start watching, and then on each window we receive from the window-ready event.

window-ready is emitted from the BrowsingContextTargetActor, via the onWindowCreated handler. This one can be triggered either by a DOMWindowCreated event or by a pageshow event.

Here what seems to happen when reloading:

• we get the top level DomWindowCreated fine
• then we get DomWindowCreated for the iframe, which are actually initial documents (isInitialDocument is true). window-ready is sent for all of those and we set the styleSheetChangeEventsEnabled on the document.
• then I guess those transient documents are "destroyed" (or swapped, not sure, but we don't get them in windowDestroyed) and the "real" one is created (we don't get a DOMWindowCreated for it though). The issue is that it has the same innerWindowId than the initial document, which means we won't get window-ready for those: https://searchfox.org/mozilla-central/rev/8f08c21f093be1c1c42438697f8bca67af94fc77/devtools/server/actors/targets/browsing-context.js#1777-1780 would see them as already handled.
• at this point, the iframe document.styleSheetChangeEventsEnabled is false and we won't get notified about its stylesheets

Patch for Bug 1724909 is adding a test (devtools/shared/commands/resource/tests/browser_resources_stylesheets_navigation.js) with disabled assertions, that should be enabled as part of this bug.

A proper solution would be to move that flag on the browsing context (we already have devtools-specific flags on it), and setting it would propagate it to the whole browsing context tree

Regression window:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b54e100794eef36af9e4cefe3b260fe9642ea3bf&tochange=ccd521ebc464ffdad1a5abfa3f03a9df603c67da

(In reply to Alice0775 White from comment #3)

Regression window:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b54e100794eef36af9e4cefe3b260fe9642ea3bf&tochange=ccd521ebc464ffdad1a5abfa3f03a9df603c67da

Thanks! I strongly suspect Bug 1625930 (turning stylesheets into resources)

Removing the regressionwindow-wanted keyword as it was provided in Comment 3.

Set release status flags based on info from the regressing bug 1625930

This should be fixed with EFT

EFT is enabled everywhere and all blockers are resolved, we can probably close this