I'd like to request we re-prioritize this, given that this is really bad now that we load a stylesheet for each instance of each component, rendering the stylesheet editor completely unusable on chrome pages.

This is a bit of WIP patch. It switches from a single styleSheet per
StyleSheetEditor to using a Set() of all the sheets with the same URI, and changes
StyleEditorUI.jsm to reuse editors based on the href of the sheet. Sheets
without a URI continue to have their own editors.

Questions I have on how to finish this:

1. what to do if the page modifies one of the sheets via CSSOM. I think
   we would get events, though I could be wrong - but in any case, the question
   is more what to do when this happens. We could try to "detach" the stylesheet
   in question again, but I'm not sure how to do that or how to avoid renewing
   the confusion this patch aims to solve (ie: how would the user know which of
   the sheets is which, if they're being modified). I'm also not sure if this is
   common enough to warrant worrying about right now.
2. what to do about source maps. I don't know how to test this well. I've
   tried to be reasonable in my changes, but chrome code doesn't use them so it's
   quite possible I've broken something without realizing. The easy way out might
   be not to unify the sheets if they're using source maps? But that'd probably
   reduce utility for web developers.
3. highlighting nodes. I'm not sure how this works right now (I couldn't
   see any highlights when mousing over the editor), and if I can invoke the
   highlighter multiple times or if there's some other way to resolve this.
4. automated tests. I assume I should add one; what would be a good template
   to base it on?

Finally, I'm really not sure who should review this - the last 3 months have
seen 3 patches to the style editor, all reviewed by pbro, who seems to have
moved on. :-(

On the plus side, this makes working with e.g. about:preferences and the browser
window reasonable again! Also, if you change a rule in one web component sheet
from the inspector or style editor, it is reflected across all instances. So
that's pretty neat.

The following patch is waiting for review from reviewers who are inactive or resigned from the review:

:Gijs, could you please find another reviewer or abandon the patch if it is no longer relevant?

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