From 8948c9ec1b8ade3d1e4ed7103fd4f6d17a8ea23e Mon Sep 17 00:00:00 2001
From: Gabriel Schneider <gbrls@0x4200.cafe>
Date: Mon, 28 Sep 2026 19:00:49 -0300
Subject: A replace through data or body, truncate then write, is one undo step

A shell's printf x > data truncates the addressed range and then writes, and each pushed its own undo point, so one replace took two Undos. A truncation of data, xdata or body now lets the write that follows it at once join its step, as acme's replace is one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 src/fs.zig         |  3 ++-
 src/ninep/pane.zig | 22 +++++++++++++++++++++-
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/src/fs.zig b/src/fs.zig
index 42d761ca..0a4473d6 100644
--- a/src/fs.zig
+++ b/src/fs.zig
@@ -598,7 +598,8 @@ test "virtual body writes can read their input from the same pane" {
     const path = try std.fmt.bufPrint(&path_buffer, "/virtual/pane/{d}/body", .{pane.serial});
     try write(p, path, pane.file.?.content);
     try std.testing.expectEqualStrings("same pane contents\n", pane.file.?.content);
-    try std.testing.expectEqual(@as(usize, 2), pane.file.?.history.undo_len);
+    // The truncation and the write are one undo step.
+    try std.testing.expectEqual(@as(usize, 1), pane.file.?.history.undo_len);
     const events = &p.panes[p.active].?.fs.events;
     try std.testing.expect(std.mem.startsWith(u8, events.peek().?, "ED"));
     events.pop();
diff --git a/src/ninep/pane.zig b/src/ninep/pane.zig
index a5acccbc..50fc220e 100644
--- a/src/ninep/pane.zig
+++ b/src/ninep/pane.zig
@@ -47,6 +47,10 @@ pub const State = struct {
     event_reader: bool = false,
     events: events.Queue = .{},
     nomark: bool = false,
+    /// The revision a truncation of `data`, `xdata` or `body` left: the
+    /// write that follows it at once joins its undo step, so a shell's
+    /// `printf x > data` is one step, as acme's replace is.
+    joined: ?u32 = null,
     noscroll: bool = false,
     tag_snap: std.ArrayList(u8) = .empty,
     /// An open reads pty/data, so output queues for it; a second is refused.
@@ -190,7 +194,9 @@ pub fn spliceBody(p: *Pardes, pane: *Pane, q0: usize, q1: usize, bytes: []const
     @memcpy(new[0..lo], f.content[0..lo]);
     @memcpy(new[lo..][0..take], bytes[0..take]);
     @memcpy(new[lo + take ..], f.content[hi..]);
-    if (!pane.fs.nomark) panes.File.pushUndo(p, pane);
+    const join = pane.fs.joined == f.revision;
+    pane.fs.joined = null;
+    if (!pane.fs.nomark and !join) panes.File.pushUndo(p, pane);
     panes.File.setContent(p, f, new);
     return take;
 }
@@ -614,6 +620,7 @@ pub fn truncate(p: *Pardes, pane: *Pane, file: PaneFile) tree.Status {
     switch (file) {
         .body => if (fileOf(pane) != null) {
             _ = spliceBody(p, pane, 0, bodyOf(pane).len, "") orelse return .err;
+            pf.joined = fileOf(pane).?.revision;
             pf.addr = .{};
             setDot(pane, .{});
         },
@@ -623,6 +630,7 @@ pub fn truncate(p: *Pardes, pane: *Pane, file: PaneFile) tree.Status {
             const q1 = @max(q0, pf.addr.q1);
             const before = dotOf(pane);
             _ = spliceBody(p, pane, q0, q1, "") orelse return .err;
+            pf.joined = fileOf(pane).?.revision;
             setDot(pane, .{ .q0 = shiftOne(before.q0, q0, q1 - q0, 0), .q1 = shiftOne(before.q1, q0, q1 - q0, 0) });
             pf.addr = .{ .q0 = q0, .q1 = q0 };
         },
@@ -909,6 +917,18 @@ 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 "a replace through data, truncate then write, is one undo step" {
+    const p = try withFile(testing.allocator, "one two\n");
+    defer p.deinit();
+    const serial = serialOf(p);
+    _ = wr(p, Node.of(serial, .addr), "/two/");
+    _ = call(p, .{ .tag = 1, .op = .setattr, .node = Node.of(serial, .data), .truncate = true });
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .data), "2").reply.status);
+    try testing.expectEqualStrings("one 2\n", rd(p, Node.of(serial, .body), 0, 64).bytes);
+    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .ctl), "Undo").reply.status);
+    try testing.expectEqualStrings("one two\n", rd(p, Node.of(serial, .body), 0, 64).bytes);
+}
+
 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();
-- 
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