3 files changed, 55 insertions, 1 deletions

diff --git a/src/builtins.zig b/src/builtins.zig
index 6688d05f..27a1d36d 100644
--- a/src/builtins.zig
+++ b/src/builtins.zig
@@ -1271,6 +1271,9 @@ pub const Joincol = struct {
         // The keyboard's column joins the one to its right; the last has none.
         const col = c.p.keyboardColumn(c.p.active) orelse return;
         if (col + 1 >= c.p.ncol) return c.p.reportFailure(c.id, "Joincol: no column to the right");
+        // As Newcol is refused: a column that could not give each pane its
+        // tag and 2 rows would leave a layout `size` refuses as it is.
+        if (!layout.joinFits(c.p, col)) return c.p.reportFailure(c.id, "Joincol: no space: the joined column cannot give each pane its tag and 2 rows");
         layout.joinCol(c.p, col);
     }
 };
diff --git a/src/layout.zig b/src/layout.zig
index 4aebcf28..a2211dfd 100644
--- a/src/layout.zig
+++ b/src/layout.zig
@@ -360,13 +360,38 @@ pub fn noteColumn(p: *Pardes, what: []const u8, serial: u32) void {
 pub fn joinCol(p: *Pardes, c: usize) void {
     if (c + 1 >= p.ncol) return;
     const dst = c + 1;
+    joinPanes(p, c, dst);
+    dropColumn(p, c);
+}
+
+/// Whether column `c` joined into the one on its right leaves every pane
+/// its tag and 2 rows (minimaFit), tried in place and undone: nothing is
+/// dropped or logged for a join that does not fit.
+pub fn joinFits(p: *Pardes, c: usize) bool {
+    if (c + 1 >= p.ncol) return true;
+    const dst = c + 1;
+    const n = p.col_n[c];
+    const had = p.col_n[dst];
+    const weights = .{ p.col_weight[c], p.col_weight[dst] };
+    const panes_c = p.col_panes[c];
+    joinPanes(p, c, dst);
+    const fits = minimaFit(p);
+    p.col_n[dst] = had;
+    p.col_n[c] = n;
+    p.col_panes[c] = panes_c;
+    p.col_weight[c] = weights[0];
+    p.col_weight[dst] = weights[1];
+    compute(p);
+    return fits;
+}
+
+fn joinPanes(p: *Pardes, c: usize, dst: usize) void {
     for (0..p.col_n[c]) |k| p.col_panes[dst][p.col_n[dst] + k] = p.col_panes[c][k];
     p.col_n[dst] += p.col_n[c];
     p.col_n[c] = 0;
     // The width goes right, where the panes went.
     p.col_weight[dst] +|= p.col_weight[c];
     p.col_weight[c] = 0;
-    dropColumn(p, c);
 }
 
 /// Put new pane `id` in column `c` as acme's makenewwindow does
diff --git a/src/ninep/cols.zig b/src/ninep/cols.zig
index a1fe9ecc..7afcfbd8 100644
--- a/src/ninep/cols.zig
+++ b/src/ninep/cols.zig
@@ -232,6 +232,32 @@ 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 "Joincol is refused when the joined column cannot give each pane its tag and 2 rows" {
+    const p = try pardes.Pardes.init(testing.allocator, .{ .tty_only = true, .cols = 160, .rows = 12 });
+    defer p.deinit();
+    while (p.nextEffect()) |_| {}
+    const ctl = @intFromEnum(tree.TopFile.ctl);
+    const focus = @intFromEnum(tree.TopFile.focus);
+    // Two panes in the first column, then a second column of two.
+    const a = try th.newPane(p);
+    _ = a;
+    try testing.expectEqual(tree.Status.ok, th.wr(p, ctl, "Newcol\n").reply.status);
+    const b = try th.newPane(p);
+    var digits: [16]u8 = undefined;
+    _ = th.wr(p, focus, try std.fmt.bufPrint(&digits, "{d}\n", .{b}));
+    _ = try th.newPane(p);
+    try testing.expectEqual(@as(usize, 2), p.ncol);
+    try testing.expect(p.col_n[0] >= 2 and p.col_n[1] >= 2);
+    // Focus in the left column: joined, four panes want 12 rows of 10.
+    const left = p.panes[p.col_panes[0][0]].?.serial;
+    _ = th.wr(p, focus, try std.fmt.bufPrint(&digits, "{d}\n", .{left}));
+    const r = th.wr(p, ctl, "Joincol\n");
+    try testing.expectEqual(tree.Status.err, r.reply.status);
+    try testing.expect(std.mem.indexOf(u8, r.reply.ename, "tag and 2 rows") != null);
+    try testing.expectEqual(@as(usize, 2), p.ncol);
+    try testing.expect(pardes.layout.minimaFit(p));
+}
+
 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();