The paginator's scrolled-mode scroll handler is debounced 250 ms, so
#anchor and #primaryIndex can lag behind the user's actual viewport.
Toggling out of scrolled mode within that window made
render() → scrollToAnchor(#anchor) restore the stale anchor, reverting
the position to a previously visible section.
Update foliate-js to flush the pending scroll state before flow leaves
'scrolled', and add regression coverage for the multi-section toggle path.
Huang Xin