As noted in [1], today the global root scroller is recomputed after an effective root scroller changes in any frame on the page. This can now be done after effective selection inside LocalFrameView::UpdateAllLifecyclePhasesInternal

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1060032/9/third_party/blink/renderer/core/page/scrolling/README.md#49