Moves a viewport from an earlier layout of the same pages to this one (real page sizes replacing placeholders, other options)
so that it keeps showing what it showed: the current page of previous keeps its top-left corner where it was
in the viewport, at the same zoom.
public Viewport Anchor(PageLayout previous, in Viewport viewport)
The layout the viewport was in.
The viewport.
Spec #33 viewer capability 3. Without it a page that grows the column (a wider page arriving after page 1's placeholder size) would shift every page across the strip under fixed scroll offsets, and the page in view would slide out of it.