smoother scroll restore on jump-back; non-degenerate test

The restored scroll was correct in the final state but load() zeroed
virtual_size (snapping scroll to 0) and only a deferred call_after_refresh
jumped to the target -> a visible flash to the top before settling. Now: don't
zero virtual_size on load, and apply the target scroll synchronously in
_on_primed (plus the deferred call as a fallback for un-sized regions).

Also hardened the scroll test to use a MID-viewport cursor (rel>0). The prior
test had the cursor at the viewport top (rel=0), where scroll-into-view derives
the same scroll -> it would have passed even if scroll restore were broken.
pilot suite 52/52.

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