Subclasses that need a child's real measured size at gesture-time (not
the next frame) can set a per-class measure-request field and call this
to make `performLayout` run synchronously, observe the field, and
populate sizes. No extra frame scheduled.
Banlu's chat-stream uses this for scroll-to-reply on far-back history
that hasn't been laid out yet — without it, the path is either iterative
single-viewport jumps (waits for layout per jump) or minHeight estimates
that misalign the landing position.
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