Zerox opens a second pane on a file pane's buffer: one text, undo and unsaved state, a scroll and a cursor each; an edit in either is spliced into both at File.setContentSpan, O(the edit) more, and what a Save, a get, the disk watch or a rename changes on one is copied to the rest after each step

Twins share the content buffer, the History and, while it is one, the line
index; the other twin's cursor and scroll move past the splice as acme
moves a clone's q0. Del of one twin asks nothing while another holds the
text, Exit lists the text once, and a closed twin keeps a copy of its own
until it is torn down. Both are in the index, each its serial, one name.
An event reader on one hears no edit made through another (first cut).
A dump keeps the twins as two panes on the file, not linked again.
perf: edit-twin, typing beside a twin of the 50k-line file, gated within
3x of edit-char in the same run; the three baselines re-recorded for it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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