Writes made under mark 0 are one undo step, and no history is lost

mark 0 skipped the undo point of each write, and mark 1 then pushed the text as it had become, so the text from before the batch was in no step: with no history before it, no Undo brought it back. Turning mark off now keeps the text as it is, and the writes after it are one step back to it, as acme's nomark joins them.

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

1 files changed, 20 insertions, 1 deletions

diff --git a/src/ninep/pane.zig b/src/ninep/pane.zig
index 714a2566..a5acccbc 100644
--- a/src/ninep/pane.zig
+++ b/src/ninep/pane.zig
@@ -516,9 +516,12 @@ fn writeFlag(p: *Pardes, req: Req, pane: *Pane, file: PaneFile) Reply {
             const f = fileOf(pane) orelse return Reply.fail(req.tag, E.INVAL);
             f.saved_revision = if (on) f.revision -% 1 else f.revision;
         },
+        // acme's nomark joins the writes after it into one undo step. The
+        // step starts here: the text as it is now is what Undo brings back,
+        // and turning mark on again only ends it.
         .mark => {
+            if (!on and !pf.nomark) panes.File.pushUndo(p, pane);
             pf.nomark = !on;
-            if (on) panes.File.pushUndo(p, pane);
         },
         .scroll => pf.noscroll = !on,
         else => unreachable,
@@ -906,6 +909,22 @@ test "the flag files read and write the state acme kept behind ctl verbs" {
     try testing.expectEqual(E.INVAL, wr(term, Node.of(serialOf(term), .dirty), "1").errno());
 }
 
+test "writes under mark 0 are one undo step, even with no history before them" {
+    const p = try withFile(testing.allocator, "one\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    const body = Node.of(serial, .body);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .mark), "0").reply.status);
+    _ = wr(p, body, "two\n");
+    _ = wr(p, body, "three\n");
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .mark), "1").reply.status);
+    try testing.expectEqualStrings("one\ntwo\nthree\n", rd(p, body, 0, 64).bytes);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Undo").reply.status);
+    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\nthree\n", rd(p, body, 0, 64).bytes);
+}
+
 test "truncating the tag clears its editable tail" {
     const p = try withFile(testing.allocator, "x\n");
     defer p.deinit();