NSTextView (TextKit 1, allowsNonContiguousLayout) scroll jumps when re-highlighting the whole buffer per keystroke

Symptom: an editor that re-applies syntax-highlight attributes over the full NSTextStorage range after each keystroke briefly scrolls far up (about 100 lines, or to the top of a short file) and sometimes settles hundreds of points away. Cause: the full-range attribute edit invalidates layout for every line. With allowsNonContiguousLayout the unlaid line heights get re-estimated, and inside NSTextStorage endEditing the layout manager (_resizeTextViewForTextContainer) re-anchors by calling scrollRangeToVisible on the old visible range, so the clip view origin moves. Fix: compare against the existing attributes (attributes(at:longestEffectiveRange:in:) plus NSDictionary isEqual) and call setAttributes only on runs that differ. Then a keystroke edits a few characters and the clip origin never moves. Related: after a full relayout (string replace, wrap toggle), restoring the old numeric clip origin shows different lines. Anchor to a character index plus a pixel offset instead. To debug, observe NSView.boundsDidChangeNotification on the clip view and log Thread.callStackSymbols.