pane/new takes rows from a pane in another column before refusing ENOSPC, as acme's placement does

A pane/new whose column had no room was refused "no space for a pane in
that column", even with another column holding a tall pane that could
give rows. acme takes rows wherever a window has them before saying
there are none. pane/new now tries the other columns as +Errors already
did: an empty one, else its tallest pane's bottom half, last column
first. It is refused only when no pane anywhere can give the rows.

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

3 files changed, 36 insertions, 4 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 33cfd581..b37511dd 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -367,7 +367,10 @@ answer the same serial. It goes where acme's makenewwindow puts a window
 if empty, else the bottom half of its last pane. A session holds 64 panes
 (`no space for a pane: 64 max`), and every pane keeps its tag and 2 rows
 (`no space for a pane in that column: each keeps its tag and 2 rows`); both
-are ENOSPC, for this open and for a look, exec, `New` or `Tty` alike.
+are ENOSPC, for this open and for a look, exec, `New` or `Tty` alike. A
+`pane/new` whose column is full takes its rows from a pane in another column
+that has them, last column first, as acme does, and is refused only when
+no pane anywhere can give them.
 `rmdir /pane/<n>` closes the pane, unsaved or not.
 
 **`name`** reads the file name (a terminal's directory); a write renames the
diff --git a/src/exec.zig b/src/exec.zig
index 8356db75..7d682d74 100644
--- a/src/exec.zig
+++ b/src/exec.zig
@@ -1045,9 +1045,12 @@ pub fn placeNew(p: *Pardes, from: usize, origin: ?usize, id: usize, kind: NewPan
             // No pane to halve, but the column holds every pane's minimum
             // with the new one's: the rows are shared out again, so what is
             // refused is only what the documented arithmetic cannot hold.
-            // +Errors goes wherever there is room, last column first; with
-            // none, its maker says the text some other way (appendErrors).
-            if (layout.shareColumn(p, f.col)) {} else if (kind == .errors) {
+            // +Errors and 9P's pane/new go wherever there is room, last
+            // column first, as acme takes rows from a window that has them
+            // before saying there are none; with none, +Errors's maker says
+            // the text some other way (appendErrors), and pane/new is
+            // refused.
+            if (layout.shareColumn(p, f.col)) {} else if (kind == .errors or kind == .fs) {
                 layout.removePane(p, id);
                 var c = p.ncol;
                 while (c > 0) {
@@ -1064,6 +1067,11 @@ pub fn placeNew(p: *Pardes, from: usize, origin: ?usize, id: usize, kind: NewPan
                 } else {
                     layout.insert(p, f.col, p.col_n[f.col], id);
                     p.unplaced.set(id);
+                    // No pane anywhere can give it rows: pane/new is refused.
+                    if (kind == .fs) {
+                        p.panes[id].?.vweight = 0;
+                        p.reportError(from, "placement", error.NoPaneRoom);
+                    }
                     return;
                 }
             } else {
diff --git a/src/ninep/cols.zig b/src/ninep/cols.zig
index 9185a2bc..c4ee83eb 100644
--- a/src/ninep/cols.zig
+++ b/src/ninep/cols.zig
@@ -228,6 +228,27 @@ test "New written to a column's exec makes its scratch in the session's director
     try testing.expectEqualStrings("/project/src/+New", p.panes[p.active].?.file.?.path);
 }
 
+test "pane/new takes rows from a pane in another column before refusing ENOSPC" {
+    const p = try pardes.Pardes.init(testing.allocator, .{ .tty_only = true, .cols = 160, .rows = 16 });
+    defer p.deinit();
+    while (p.nextEffect()) |_| {}
+    const first = p.panes[p.active].?.serial;
+    // A second column holding one tall pane.
+    try testing.expectEqual(tree.Status.ok, th.wr(p, @intFromEnum(tree.TopFile.ctl), "Newcol\n").reply.status);
+    const tall = try th.newPane(p);
+    const tall_col = layout.findPane(p, p.paneBySerial(tall).?).?.col;
+    const focus = @intFromEnum(tree.TopFile.focus);
+    var digits: [16]u8 = undefined;
+    // From the first column, until pane/new is refused.
+    var moved = false;
+    for (0..pardes.MAX_PANES) |_| {
+        _ = th.wr(p, focus, try std.fmt.bufPrint(&digits, "{d}\n", .{first}));
+        const made = th.newPane(p) catch break;
+        if (layout.findPane(p, p.paneBySerial(made).?).?.col == tall_col) moved = true;
+    }
+    try testing.expect(moved);
+}
+
 test "a Newcol refused because the tags would not fit logs its err alone and spends no column serial" {
     const p = try pardes.Pardes.init(testing.allocator, .{ .tty_only = true, .cols = 160, .rows = 14 });
     defer p.deinit();