Two opens writing one body are separate undo steps, however they interleave

The join mark a write left was the pane's, so another open's write joined the first open's step. Writing opens of body, data and xdata now get an open record, the mark names the open that set it, and another open's write starts its own step (the pane's one history: each turn the opens take is a step).

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

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index dc15ab91..03cdd1ee 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -738,7 +738,9 @@ The three flag files `dirty`, `mark` and `scroll` read `0` or `1` and take
 an undo point (writing `1` pushes one now), and whether a write scrolls the
 pane. The writes of one open of `data`, `xdata` or `body` are one undo
 step while `mark` is 1 (so `printf 'a\nb\n' > data` is one, though a
-shell writes it a line at a time), and the next open starts another; to
+shell writes it a line at a time), and the next open starts another. The
+pane has one history, so two opens writing at once take turns in it: each
+open's first write after the other's starts a step of its own; to
 make a loop's writes one step, write `1` (an undo point here), then `0`,
 the writes, then `1` again.