We introduce `::-moz-snapshot-containing-block` pseudo-element to wrap
the view transition pseudo-element tree. This pseudo-element is only for
internal usage and cannot be accessed from outside.
Note that the related wpt,
`snapshot-containing-block-static.html`
`snapshot-containing-block-absolute.html`
are still failed because of Bug 1960755.
Also, add a test to check the snapshot containing block in the iframe,
and avoid hitting the issue of Bug 1960755.
There is no better way to verify that we do establish the fixed
positioning containing block, because it's unlikely to make the size of the
snapshot containing block be different from the viewport size on desktop
(note: this pseudo-element doesn't expose to the web), so I verified this
patch by changing the size of snapshot containing block manually in the UA
stylesheet, to see if it establish the fixed positioning containing block.
Differential Revision: https://phabricator.services.mozilla.com/D245648
6167f461bd