A command line runs as its own command pane unless it is clicked at a shell's prompt

A middle click, an exec write or a tag word that no builtin knows was typed
into some terminal for the pane's directory, sharing whatever state that
shell was in and answering nothing, so a misspelling vanished into a shell.
Now only a line clicked at an interactive terminal's prompt is typed there.
From anywhere else it runs as a command pane: a terminal whose child is
$SHELL -c the line in the pane's directory, full emulation, which shows its
output and then exit N from the host's reaping of the child, and stays. A
finished command pane is the next command's for its directory, which runs
below what it showed after a '% line' line (acme appends to +Errors and never
clears it, util.c:213); a running one gets a second pane. Kill ends a command
pane's whole process group, the log records run and exit, exec reads back the
command pane's serial, and a line is at most 1 KB, read off the pane as the
host forks rather than carried in every spawn effect. ttyForDir's search for
a free shell is gone. The goldens of chordcut, cmdword and layout-open change
where a file's exec now opens a command pane, and ttytaken is rewritten to
exec from the terminal itself; docs/open-questions.md records the decision.

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

1 files changed, 16 insertions, 1 deletions

diff --git a/docs/open-questions.md b/docs/open-questions.md
index db491213..a2ce2086 100644
--- a/docs/open-questions.md
+++ b/docs/open-questions.md
@@ -6,7 +6,22 @@ picked up without redoing the research.
 ## Where does an unknown command word run?
 
 Raised 2026-09-27 in the review of pardes against acme (Plan 9 source at
-`~/05-genizah/principia-softwarica`). Undecided.
+`~/05-genizah/principia-softwarica`).
+
+**Decided 2026-09-28: a command pane.** Clicked or written at an interactive
+terminal at its prompt, the line is typed into that shell, whose state the
+clicker can see. From anywhere else it runs as its own terminal pane whose
+child is `$SHELL -c` the line in the pane's directory: full emulation
+(colours, `less`, `vim`, `sudo`'s prompt work), the exit status from waiting
+on the child (`exit N`, `exit 127` for a misspelling, no prompt marks
+needed), Kill signalling its process group, `run`/`exit` records in the log,
+and `exec` answering its serial. One command pane per directory is reused
+once done, and keeps what it showed, as acme appends to `+Errors` and never
+clears it (util.c:213-221). Chosen over acme's process-into-`+Errors` because
+it keeps interactive programs working without a streaming runner, and over
+a PATH check before typing into a shell, which misjudges builtins, aliases
+and functions and leaves the shell's state in the way. The analysis below is
+what it was decided from.
 
 **Today.** A middle-click, an `exec` write or a tag word that is not a builtin
 is typed into a terminal pane: the pane itself when it takes a command line,