layout: Don't use the existing block position as the float ceiling when placing block formatting contexts in the sequential fallback.
The existing block position isn't yet computed at that time, so it
contains junk data. It just so happened to work on first reflow because
that value is usually set to zero, but it usually failed on subsequent
reflows.
Improves certain Wikipedia pages.
Closes#13630 (though Google is still broken; it was a separate bug and
will be split off into a separate issue).
r? @notriddle
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cad5a4e326
