A new pane with no room under the last goes last all the same

A pane from pane/new or New takes the bottom half of its column's last
pane, as acme's coladd does (cols.c:62-65); when that pane is too short
to give it a tag and 2 rows, the fallback halved the tallest pane and
put the new one under it, mid-column (the 7 9 8 found). It now goes last
all the same, with half the tallest pane's rows (its tag measured at the
column's width). Tty from a pane's ctl already follows acme's
makenewwindow (the active column first, util.c:456-467); fs.md now says
so rather than beside that pane.

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

3 files changed, 29 insertions, 6 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 45a64e44..5c2486f4 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -124,7 +124,9 @@ Existing Plan9port/v9fs clients need a userspace bridge for QUIC.
 /col/<n>/ctl write Delcol, Joincol, New or Tty: each acts on that column, as from its tag
 /col/<n>/exec write a word: a middle click on it in that column's tag; read as /exec; rmdir col/<n> closes
              an empty column (a column with panes is refused, ENOTEMPTY)
-/pane/new    open it to make a pane; the read answers that pane's serial. A session holds
+/pane/new    open it to make a pane (the bottom half of the active column's last
+             pane, acme's coladd; with no room there, last all the same, taking half
+             the tallest pane's rows); the read answers that pane's serial. A session holds
              64 panes (16 on the board); at that, every route that would open one -- this
              open, look, exec, New, Tty -- fails with `no space for a pane: 64 max` (ENOSPC through 9ns, which has
              no word for ENFILE) and an err
@@ -327,7 +329,10 @@ same tree without leaving the process.
   `src/builtins.zig` (`Save`, `Del`, `New`, `Newcol`, `Mount NAME DIAL`,
   `Unmount NAME`, `Dump`, `Restore`, `Msg TEXT`, `Find`, `Grep`, `Tty`, ...;
   `Tty`'s argument is the shell it runs, `Tty fish`, and `Tty` on a pane's
-  ctl opens a new terminal pane beside that one, not in it),
+  ctl opens a new terminal pane, not in that one: in the active column, as
+  acme's makenewwindow puts a new window (util.c:456-467: the active column
+  first, then the pane's own), under the text with room or halving the
+  tallest, never shorter than its tag and 2 rows),
   or anything else, a command line. Written at a terminal at its prompt it
   is typed into that shell (and a terminal whose shell exits, `exit` typed
   or run, closes its pane). From anywhere else -- a file, a scratch, a tag,
diff --git a/src/layout.zig b/src/layout.zig
index bdef9675..644054ed 100644
--- a/src/layout.zig
+++ b/src/layout.zig
@@ -447,8 +447,9 @@ pub fn everyPaneRoomy(p: *Pardes) bool {
     return true;
 }
 
-/// Places `id` in column `c` by halving its tallest unfolded pane, when
-/// that leaves both their rows; false, and `id` in no column, when not.
+/// Places `id` last in column `c`, where acme's coladd puts a new window,
+/// with half the rows of the column's tallest unfolded pane, when that
+/// leaves both their rows; false, and `id` in no column, when not.
 pub fn splitTallest(p: *Pardes, c: usize, id: usize) bool {
     compute(p);
     var tall: ?usize = null;
@@ -457,8 +458,15 @@ pub fn splitTallest(p: *Pardes, c: usize, id: usize) bool {
         if (tall == null or p.rects[k].h > p.rects[tall.?].h) tall = k;
     }
     const v = tall orelse return false;
-    if (p.rects[v].h < minRows(p, v) + minRows(p, id)) return false;
-    splitAt(p, c, id, p.rects[v].y + p.rects[v].h / 2);
+    const h = p.rects[v].h;
+    // Its tag wraps at the column's width: measured there, not at none.
+    p.rects[id].w = p.col_w[c];
+    if (h < minRows(p, v) + minRows(p, id)) return false;
+    snapColWeights(p, c);
+    insert(p, c, p.col_n[c], id);
+    const given = std.math.clamp(h / 2, minRows(p, id), h - minRows(p, v));
+    p.panes[v].?.vweight = @floatFromInt(h - given);
+    p.panes[id].?.vweight = @floatFromInt(given);
     return true;
 }
 
diff --git a/src/ninep/tree.zig b/src/ninep/tree.zig
index 27091112..af1c589d 100644
--- a/src/ninep/tree.zig
+++ b/src/ninep/tree.zig
@@ -1560,3 +1560,13 @@ test "an a, c or i text cut short through exec is an err in the log, not a msg"
     try testing.expect(!th.logHas(p, "msg 1 Edit: a, c or i"));
     try testing.expectEqualStrings("abc\n", p.panes[0].?.file.?.content);
 }
+
+test "a pane made from pane/new goes last in its column, even when the last pane has no room to halve" {
+    const p = try th.withFile(testing.allocator, "x\n");
+    defer p.deinit();
+    for (0..3) |_| {
+        const made = call(p, .{ .tag = 1, .op = .open, .node = @intFromEnum(TopFile.new) }).reply.handle;
+        p.sync();
+        try testing.expectEqual(made, p.panes[p.col_panes[0][p.col_n[0] - 1]].?.serial);
+    }
+}