Forced foreground repaints asked xterm for rows 0..rows-1. xterm's render
debouncer unions ranges, so one full-grid request widened every frame to a
whole-viewport `_updateModel` cell walk even when the write changed five rows.
Re-issue the repair over the parse's own dirty span instead, keeping the
whole grid for viewport scroll, alternate-screen flips, and any write whose
span cannot be observed.
Neil
d4fa091714