A 9P request that fills a stillborn scratch's column replaces the scratch before it is answered

A pane that joins a column holding only an empty +New takes the column, but
that replacement ran at the end of the editor's next update, not of a 9P
request: a look answered 19 while index still listed the +New 18, whose del
came later. serveFs, a step of its own, now runs it too before answering.

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

2 files changed, 26 insertions, 0 deletions

diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index deb64565..3690ac47 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -988,3 +988,25 @@ test "Joincol with no column to the right and Theme with no such theme say so" {
     const pane = p.panes[p.active].?;
     try testing.expect(std.mem.indexOf(u8, pane.msg[0..pane.msg_len], "no theme \"drak\"") != null);
 }
+
+test "a look that fills a column's stillborn scratch is answered with the scratch already gone" {
+    const gpa = testing.allocator;
+    const p = try withFile(gpa, "x\n");
+    defer p.deinit();
+    var tmp = testing.tmpDir(.{});
+    defer tmp.cleanup();
+    try tmp.dir.writeFile(testing.io, .{ .sub_path = "f.txt", .data = "file\n" });
+    var dir: [4096]u8 = undefined;
+    const at = dir[0..try tmp.dir.realPath(testing.io, &dir)];
+    var line: [4200]u8 = undefined;
+    // Newcol makes a column of one empty +New; a file looked from it
+    // joins that column and takes the scratch's place.
+    try testing.expectEqual(tree.Status.ok, wr(p, @intFromEnum(tree.TopFile.ctl), "Newcol\n").reply.status);
+    const scratch = p.panes[p.active].?.serial;
+    try testing.expectEqual(tree.Status.ok, wr(p, Node.of(scratch, .look), try std.fmt.bufPrint(&line, "{s}/f.txt\n", .{at})).reply.status);
+    try testing.expect(p.paneBySerial(scratch) == null);
+    var want: [32]u8 = undefined;
+    const gone = try std.fmt.bufPrint(&want, "{d} ", .{scratch});
+    var rows = std.mem.splitScalar(u8, rd(p, @intFromEnum(tree.TopFile.index), 0, 4096).bytes, '\n');
+    while (rows.next()) |row| try testing.expect(!std.mem.startsWith(u8, row, gone));
+}
diff --git a/src/pardes.zig b/src/pardes.zig
index 43d8e94d..12de47fa 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -4999,6 +4999,10 @@ pub const Pardes = struct {
         look.cancelLookHover(p);
         const reply = ctlfs.handle(p, req);
         if (reply.status == .err and (req.op == .write or req.op == .setattr)) ctlfs.events.noteError(p, req, reply);
+        // A pane that joined a column holding only a stillborn scratch takes
+        // it now, before the answer: one read of index after the look shows
+        // the replacement done, and the log has its del with the new.
+        exec.replaceStillborn(p);
         // The request was a whole step of its own, so it settles the way a
         // step does: the cursor and scroll reconciled, the scripted panes
         // told, and the panes it made announced to /log now rather than at