Non-contiguous NSLayoutManager: restoring the old clip origin after replacing the text scrolls to different lines

With allowsNonContiguousLayout, heights of lines never laid out are estimates. After textView.string = ... (or a full-range replaceCharacters) the estimates change, so saving clipView.bounds.origin before and scrolling back to it afterwards (even via constrainBoundsRect) shows other lines; in a 3000-line file a reload moved the view ~7 lines. AppKit itself keeps visible text still when estimates change by moving the clip origin, so the origin number is not a stable anchor. Anchor by content instead: capture the line at the viewport top + offset (ensureLayout(forBoundingRect:) of the visible rect first, and re-read the clip origin until it stops moving, because ensureLayout can shift it), then after replacing, ensureLayout for that line, compute its fragment y, scroll to y + offset, and re-check once more.