Under pardes's placement a pane whose neighbour is in no column goes at a column's foot, not nowhere

After Delcol took the last column's panes, pane/new under Placement made a scratch that no column held, since a scratch was placed only under the asking pane, which was gone; a look from it then unwrapped the missing parent in belowParent and panicked (the 9P monkey's crash-e1873f34). Both now fall back to the foot of the active column, else the last.

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

2 files changed, 31 insertions, 2 deletions

diff --git a/src/exec.zig b/src/exec.zig
index 87f6da40..2c42389c 100644
--- a/src/exec.zig
+++ b/src/exec.zig
@@ -1150,7 +1150,7 @@ fn pardesPlace(p: *Pardes, from: usize, id: usize, kind: NewPane) void {
         .scratch, .fs => if (layout.findPane(p, from)) |sf| {
             layout.insert(p, sf.col, sf.idx + 1, id);
             layout.splitBelow(p, from, nt);
-        },
+        } else atFoot(p, id),
         .shell => belowParent(p, from, id),
         .doc, .errors => pardesDoc(p, from, id),
         .command => {
@@ -1160,9 +1160,18 @@ fn pardesPlace(p: *Pardes, from: usize, id: usize, kind: NewPane) void {
     }
 }
 
+/// Where a pane goes when the one it would go under is in no column (Delcol
+/// took them all, or it was never placed): the foot of the active column,
+/// else of the last.
+fn atFoot(p: *Pardes, id: usize) void {
+    const c = p.activeColumn() orelse p.ncol - 1;
+    layout.insert(p, c, p.col_n[c], id);
+    if (p.col_n[c] == 1) p.panes[id].?.vweight = 1;
+}
+
 fn belowParent(p: *Pardes, from: usize, id: usize) void {
     const parent = layout.splitParent(p, from);
-    const f = layout.findPane(p, parent).?;
+    const f = layout.findPane(p, parent) orelse return atFoot(p, id);
     layout.insert(p, f.col, f.idx + 1, id);
     layout.splitBelow(p, parent, p.panes[id].?);
 }
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index b9268208..59a80f36 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -1752,6 +1752,26 @@ test "a look after Newcol fills the empty column the keyboard is on" {
     try testing.expectEqual(p.active, p.col_panes[p.ncol - 1][0]);
 }
 
+test "a look under pardes's placement, from a pane pane/new made after Delcol, opens its file" {
+    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;
+    const first = serialOf(p);
+    try testing.expectEqual(tree.Status.ok, wr(p, Node.of(first, .exec), "Placement\n").reply.status);
+    try testing.expectEqual(tree.Status.ok, wr(p, Node.of(first, .exec), "Delcol\n").reply.status);
+    const made = try th.newPane(p);
+    try testing.expectEqual(tree.Status.ok, wr(p, Node.of(made, .look), try std.fmt.bufPrint(&line, "{s}/f.txt:#4\n", .{at})).reply.status);
+    // The pane pane/new made is placed, and so is the one its look opened.
+    for (p.panes, 0..) |slot, i| if (slot != null) try testing.expect(layout.findPane(p, i) != null);
+    try testing.expect(p.panes[p.active].?.serial != made);
+}
+
 test "a Restore of a file that is no dump fails the write before any warning, and a failed Dump fails its write" {
     if (comptime !pardes.hosted) return error.SkipZigTest;
     const p = try withFile(testing.allocator, "x\n");