A focus write makes its pane's column the active one, and layout's active is it

Writing a serial to /focus gave that pane the keyboard but left the active
column where it was, so pane/new under Placement acme went elsewhere, and
/layout's active named the keyboard's column rather than where a new pane
would go. A focus write now sets the active column as the user's click
there would, under either Placement, and /layout's active names the active
column, which pane/new and a look place into.

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

3 files changed, 31 insertions, 5 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 55660bc0..a5d3c4ca 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -102,11 +102,13 @@ Existing Plan9port/v9fs clients need a userspace bridge for QUIC.
              write follow to that open to wait for more
 /screen      rendered screen JSON; frozen per open handle
 /listeners   the session's dial addresses
-/focus       the serial of the pane with the keyboard; write a serial to give it the keyboard
+/focus       the serial of the pane with the keyboard; write a serial to give it the keyboard,
+             which makes its column the active one as a click there would
 /ctl         the settings, one a line as a write takes them; write a setting or a session builtin
 /commands    every builtin: word, `arg` if it takes one, `root` or `pane` (the ctl that takes it), a setting's values
 /layout      one line per column, left to right: serial index x width current|notcurrent
-             empty|full pane-serials...; then active <serial> (- when no column has the keyboard)
+             empty|full pane-serials...; then active <serial>: the active column, where
+             pane/new and a look place a pane next (- when there is none)
 /tag         the workspace tag; > replaces it, >> appends, one line
 /col/<n>/tag the tag of the column with serial n, the same way
 /pane/new    open it to make a pane; the read answers that pane's serial. A session holds
diff --git a/src/ninep/cols.zig b/src/ninep/cols.zig
index 682c2a02..79943b1b 100644
--- a/src/ninep/cols.zig
+++ b/src/ninep/cols.zig
@@ -27,7 +27,8 @@ pub fn activeColumn(p: *const Pardes) ?usize {
 /// it, which stays while it lives), its index left to right, x and width in
 /// cells, `current` or `notcurrent` (rio's words, for the column with the
 /// keyboard), `empty` or `full`, and its panes' serials, top to bottom;
-/// then `active <serial>`.
+/// then `active <serial>`: the active column, where pane/new and a look
+/// place a pane next (acme's activecol), not always the keyboard's.
 pub fn writeLayout(p: *Pardes, w: *std.Io.Writer) !void {
     const active = activeColumn(p);
     for (0..p.ncol) |c| {
@@ -38,7 +39,7 @@ pub fn writeLayout(p: *Pardes, w: *std.Io.Writer) !void {
         for (p.col_panes[c][0..p.col_n[c]]) |id| if (p.panes[id]) |pane| try w.print(" {d}", .{pane.serial});
         try w.writeByte('\n');
     }
-    if (active) |c| try w.print("active {d}\n", .{layout.columnSerial(p, c)}) else try w.writeAll("active -\n");
+    if (p.activeColumn() orelse active) |c| try w.print("active {d}\n", .{layout.columnSerial(p, c)}) else try w.writeAll("active -\n");
 }
 
 pub fn layoutLen(p: *Pardes) u64 {
@@ -179,3 +180,23 @@ test "a column's tag and the workspace's read, replace and append as a pane tag
     _ = th.wr(p, top, "Newcol Exit\n");
     try testing.expectEqualStrings("Newcol Exit", tagline.topbar(p));
 }
+
+test "a focus write makes its pane's column the active one: layout says so and pane/new goes there" {
+    for ([_][]const u8{ "Placement acme", "Placement pardes" }) |placement| {
+        const p = try th.withFile(testing.allocator, "x\n");
+        defer p.deinit();
+        try testing.expect(p.executeBuiltinLine(p.active, placement));
+        const left = p.panes[0].?.serial;
+        p.newColumn(p.active);
+        p.newScratchFor(p.active, .fs); // fills the new, right column
+        p.sync();
+        try testing.expectEqual(@as(usize, 1), layout.findPane(p, p.active).?.col);
+        var digits: [16]u8 = undefined;
+        _ = th.wr(p, @intFromEnum(tree.TopFile.focus), try std.fmt.bufPrint(&digits, "{d}\n", .{left}));
+        var want: [32]u8 = undefined;
+        const text = th.rd(p, @intFromEnum(tree.TopFile.layout), 0, 4096).bytes;
+        try testing.expect(std.mem.endsWith(u8, text, try std.fmt.bufPrint(&want, "active {d}\n", .{layout.columnSerial(p, 0)})));
+        const made = th.call(p, .{ .tag = 1, .op = .open, .node = @intFromEnum(tree.TopFile.new) }).reply.handle;
+        try testing.expectEqual(@as(usize, 0), layout.findPane(p, p.paneBySerial(made).?).?.col);
+    }
+}
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index 325fc8c8..98120555 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -486,9 +486,12 @@ pub fn writeFocus(p: *Pardes, req: Req) Reply {
     const text = std.mem.trim(u8, req.data, " \t\r\n");
     const serial = std.fmt.parseInt(u32, text, 10) catch return tree.failText(req.tag, E.INVAL, tree.e_bad_ctl);
     const id = p.paneBySerial(serial) orelse return tree.failText(req.tag, E.NOENT, e_no_pane);
-    // The keyboard goes to the pane itself, off any header that had it.
+    // The keyboard goes to the pane itself, off any header that had it, and
+    // its column becomes the active one, as the user's click there would
+    // make it: pane/new and a look place there next, whatever Placement.
     tagline.exitHeader(p);
     p.active = id;
+    if (pardes.layout.findPane(p, id)) |at| p.setActiveColumn(at.col);
     return .{ .tag = req.tag, .written = @intCast(req.data.len) };
 }