We share the inner object across sheets from the same URL, so what happens here
is that, once the sheet parses and loads, we call SetEnabled() on the first
sheet, which sets the inner bit, then calls ApplicableStateChanged.
That set actually turned the second sheet complete, so when inserting the sheet,
we think that the second sheet is already enabled, and thus in the author data,
and try to insert before it. Of course there's nothing there, so we panic.
We rely on calling SetEnabled() on all the sheets already to insert them in the
styleset / author data, so this makes it clearer and fixes the bug by moving the
state to each individual sheet.
Differential Revision: https://phabricator.services.mozilla.com/D3798
--HG--
extra : moz-landing-system : lando
