There's two potential cases handled by this patch:
(1) A scrollbar container followed by another scrollbar container.
In this case, we need to ensure these are placed into separate
clusters, even though the cluster flags otherwise match, to
ensure that slice creation will see the two clusters.
(2) If a fixed position scroll root trails a scrollbar container.
In this case, ensure that a scrollbar contains marks the
cluster flags to create a slice straight after the scrollbar,
to avoid other primitives with the same scroll root sneaking
into the scrollbar container.
Differential Revision: https://phabricator.services.mozilla.com/D61519
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extra : moz-landing-system : lando
adbb5ec13d