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, 5 insertions, 0 deletions

diff --git a/src/builtins.zig b/src/builtins.zig
index 51771c8f..8c6228c0 100644
--- a/src/builtins.zig
+++ b/src/builtins.zig
@@ -959,14 +959,19 @@ pub const Edit = struct {
 
 /// acme's Undo and Redo (exec.c `undo`): step the pane's body back or
 /// forward through its edits, from a tag, a pane ctl or exec.
+/// With no step left, Undo and Redo say so and change nothing; a ctl write
+/// of them still succeeds, as acme's Undo is silent there. A file keeps
+/// limits.undo_max steps (256).
 pub const Undo = struct {
     pub fn run(c: Ctx) void {
+        if (c.pane.file) |f| if (f.history.undo_len == 0) return c.p.setMessage(c.id, "Undo: nothing to undo");
         edit.doUndo(c.p, &c.pane.body);
     }
 };
 
 pub const Redo = struct {
     pub fn run(c: Ctx) void {
+        if (c.pane.file) |f| if (f.history.redo_len == 0) return c.p.setMessage(c.id, "Redo: nothing to redo");
         edit.doRedo(c.p, &c.pane.body);
     }
 };