Undo and Redo with nothing left say so, and the write succeeds

A pane's Undo or Redo with no step left did nothing and said nothing. It
now says "Undo: nothing to undo" (or Redo's) as a message, not a failure,
so a ctl write of it still succeeds as acme's silent Undo does. The docs
give the 256-step history.

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

1 files changed, 6 insertions, 0 deletions

diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index 0f22614c..d6922390 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -724,6 +724,12 @@ test "Undo and Redo on a pane's ctl step its body through its edits" {
     try testing.expectEqualStrings("one\n", rd(p, body, 0, 64).bytes);
     try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Redo").reply.status);
     try testing.expectEqualStrings("one\ntwo\n", rd(p, body, 0, 64).bytes);
+    // Nothing left: said, and the write still succeeds, as acme's is silent.
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Redo").reply.status);
+    try testing.expect(th.logHas(p, "Redo: nothing to redo"));
+    _ = wr(p, Node.of(serial, .ctl), "Undo");
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Undo").reply.status);
+    try testing.expect(th.logHas(p, "Undo: nothing to undo"));
 }
 
 test "focus reads the pane with the keyboard, and a serial written to it moves the keyboard there" {