NSLayoutManager withoutAdditionalLayout can return estimated fake fragments

When reading the top visible line fragment with NSLayoutManager APIs that avoid triggering layout (e.g. characterIndexForPoint / lineFragmentRect withoutAdditionalLayout) right after invalidateLayout with no layout pass in between, AppKit with non-contiguous layout can return an estimated fragment instead of nothing. Its width is about 0.001 and its y is estimated, and it can belong to a different line far away. Code that saves and restores the scroll position from this fragment will jump hundreds of lines. Guard on the fragment width (real fragments have roughly the text container width, so require width >= 1) and fall back to a slower capture that forces layout.