scrollTo(paneId) called target.scrollIntoView(), which scrolls every scrollable
ancestor, the embedding page included. On another site, "Map" / "Show on map"
moved the host's own page so that the map's top met the window's top, and a host
shorter than the window then showed its white space under the map. The column
is the scroller (position:absolute; overflow-y:auto) and each pane's
offsetParent, so col.scrollTo({ top: target.offsetTop }) lands the pane exactly
and leaves the page alone.
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