@@ -1,7 +1,9 @@
 //! The commands just run on one document (`Host.activity`), shown small at the bottom right of
 //! its pane as they run: each fades in, stays a moment and fades out, the older ones rising as
 //! newer ones come. Drawn on a layer that takes no input, so it never covers the work, and on wall
-//! time, so whatever runs the commands (a person, an agent, a tape) shows the same.
+//! time, so whatever runs the commands (a person, an agent, a tape) shows the same. One that
+//! offers an action ("Undid Claude's Draw" with its Redo) takes clicks on its button only, and
+//! stays longer to be reached.
 const ActivityFeed = @This();
 
 const std = @import("std");
@@ -16,6 +18,8 @@ seen: u64 = 0,
 
 const max = 5;
 const life_ns: i128 = 2400 * std.time.ns_per_ms;
+/// An item with an action stays this long.
+const action_life_ns: i128 = 6000 * std.time.ns_per_ms;
 const fade_ns: i128 = 250 * std.time.ns_per_ms;
 const pad: f32 = 8;
 const gap: f32 = 4;
@@ -24,6 +28,19 @@ const Item = struct {
     title: [48]u8,
     len: u8,
     born: i128,
+    action: ?Action = null,
+
+    fn life(self: Item) i128 {
+        return if (self.action != null) action_life_ns else life_ns;
+    }
+};
+
+/// A button on an item: its label, and the caller's own tag for it, which `draw` answers when
+/// it is pressed.
+const Action = struct {
+    label: [16]u8,
+    len: u8,
+    tag: u64,
 };
 
 /// A feed showing only what runs from now on.
@@ -57,14 +74,26 @@ pub fn push(self: *ActivityFeed, title: []const u8, now: i128) void {
     self.len += 1;
 }
 
-/// Draw the feed in the bottom right of `area` (physical), newest lowest.
-pub fn draw(self: *ActivityFeed, area: dvui.Rect.Physical, id_extra: usize, now: i128) void {
-    // The oldest go first.
-    while (self.len > 0 and now - self.items[0].born > life_ns) {
-        std.mem.copyForwards(Item, self.items[0 .. self.len - 1], self.items[1..self.len]);
-        self.len -= 1;
+/// `push`, with a button labelled `label` on it: `draw` answers `tag` when it is pressed.
+pub fn pushAction(self: *ActivityFeed, title: []const u8, label: []const u8, tag: u64, now: i128) void {
+    self.push(title, now);
+    var action: Action = .{ .label = undefined, .len = @intCast(@min(label.len, 16)), .tag = tag };
+    @memcpy(action.label[0..action.len], label[0..action.len]);
+    self.items[self.len - 1].action = action;
+}
+
+/// Draw the feed in the bottom right of `area` (physical), newest lowest. Answers the tag of the
+/// action pressed this frame, if one was.
+pub fn draw(self: *ActivityFeed, area: dvui.Rect.Physical, id_extra: usize, now: i128) ?u64 {
+    // Those whose time is up go.
+    var kept: usize = 0;
+    for (self.items[0..self.len]) |item| {
+        if (now - item.born > item.life()) continue;
+        self.items[kept] = item;
+        kept += 1;
     }
-    if (self.len == 0) return;
+    self.len = kept;
+    if (self.len == 0) return null;
     dvui.refresh(null, @src(), null);
 
     const r = area.toNatural();
@@ -89,9 +118,10 @@ pub fn draw(self: *ActivityFeed, area: dvui.Rect.Physical, id_extra: usize, now:
         const age = now - item.born;
         // In, then out, over `fade_ns` each; rising into place as it comes in.
         const in = @min(1, @as(f32, @floatFromInt(age)) / @as(f32, @floatFromInt(fade_ns)));
-        const out = @min(1, @as(f32, @floatFromInt(life_ns - age)) / @as(f32, @floatFromInt(fade_ns)));
+        const out = @min(1, @as(f32, @floatFromInt(item.life() - age)) / @as(f32, @floatFromInt(fade_ns)));
         const alpha = @max(0, @min(in, out));
-        const w = font.textSize(item.title[0..item.len]).w + pad * 2;
+        const action_w = if (item.action) |a| font.textSize(a.label[0..a.len]).w + pad * 2 else 0;
+        const w = font.textSize(item.title[0..item.len]).w + pad * 2 + action_w;
         const rect: dvui.Rect = .{ .x = r.w - 12 - w, .y = bottom - h + (1 - in) * h * 0.5, .w = w, .h = h };
         // Glass has no fade: it grows in from, and shrinks out to, a little smaller.
         const k = 0.6 + 0.4 * alpha;
@@ -102,6 +132,7 @@ pub fn draw(self: *ActivityFeed, area: dvui.Rect.Physical, id_extra: usize, now:
         bottom -= (h + gap) * in;
     }
     core.dialogs.glassBubbles(layer.data().id, bubbles[0..self.len], rs.s, .{});
+    var pressed: ?u64 = null;
     for (self.items[0..self.len], rects[0..self.len], alphas[0..self.len], 0..) |item, rect, alpha, n| {
         dvui.labelNoFmt(@src(), item.title[0..item.len], .{}, .{
             .rect = rect,
@@ -111,5 +142,21 @@ pub fn draw(self: *ActivityFeed, area: dvui.Rect.Physical, id_extra: usize, now:
             .margin = .{},
             .color_text = .{ .color = ink.opacity(0.85 * alpha) },
         });
+        const a = item.action orelse continue;
+        // The button alone takes input, on a layer of its own over the feed's.
+        const bw = font.textSize(a.label[0..a.len]).w + pad * 2;
+        var hit: dvui.FloatingWidget = undefined;
+        hit.init(@src(), .{}, .{ .rect = .{ .x = r.x + rect.x + rect.w - bw, .y = r.y + rect.y, .w = bw, .h = rect.h }, .id_extra = id_extra *% 8 +% n });
+        defer hit.deinit();
+        if (dvui.button(@src(), a.label[0..a.len], .{}, .{
+            .expand = .both,
+            .font = font,
+            .padding = .{ .x = pad, .y = pad / 2, .w = pad, .h = pad / 2 },
+            .margin = .{},
+            .background = false,
+            .border = .{},
+            .color_text = .{ .color = dvui.themeGet().color(.highlight, .fill).opacity(alpha) },
+        })) pressed = a.tag;
     }
+    return pressed;
 }
@@ -2978,7 +2978,7 @@ fn drawDocSurface(ctx: ?*anyopaque) anyerror!dvui.App.Result {
         if (claim_pill == null) if (ds.transport.place(area, now)) |bar| {
             area.h = @min(area.h, bar.y - area.y + 6 * dvui.windowRectScale().s);
         };
-        ds.feed.draw(area, @truncate(ds.doc_id), now);
+        if (ds.feed.draw(area, @truncate(ds.doc_id), now)) |step| editor.redoFromFeed(doc, step);
     }
 
     // A menu inside the document only when its owner names one. The context is created *after*
@@ -6189,19 +6189,74 @@ pub fn redo(editor: *Editor) !void {
 
 /// An undo or a redo of the active document, through its owner's command: its transport shows,
 /// and a key held down goes further each press (`HistoryTransport.press`), the command run that
-/// many times. Nothing to undo, or a claimed document: the command as ever, and no bar.
-fn historyStep(editor: *Editor, action: enum { undo, redo }) !void {
+/// many times. Nothing to undo, or a claimed document: the command as ever, and no bar. An undo of
+/// a step someone else made says so on the document, with a Redo (`ActivityFeed.pushAction`), in
+/// place of the plain "Undo" its run would show.
+fn historyStep(editor: *Editor, action: HistoryAction) !void {
     const doc = editor.activeDoc() orelse return;
     const name = @tagName(action);
-    const shows = editor.app.host.documentClaim(doc.id) == null and editor.activeDocCommandEnabled(name);
+    const host = &editor.app.host;
+    const shows = host.documentClaim(doc.id) == null and editor.activeDocCommandEnabled(name);
     const ds = if (shows) editor.doc_surfaces.get(doc.id) else null;
     const by = if (ds) |d| d.transport.press(dvui.frameTimeNS(), action == .redo) else 1;
     for (0..by) |_| {
         if (!editor.activeDocCommandEnabled(name)) break;
+        // Who made a step is kept only once someone besides the person has acted on it.
+        const others: ?StepToTake = if (action == .undo and host.step_authors.contains(doc.id)) blk: {
+            const t = editor.stepToTake(doc, .undo, host.arena()) catch break :blk null;
+            break :blk if (t.by.kind == .person) null else t;
+        } else null;
+        const t = others orelse {
+            try editor.runActiveDocCommand(name);
+            continue;
+        };
+        // The run's own "Undo" goes unrecorded (as while a demo seeks), so the bubble below is
+        // the one it shows. The flag is put back before the feed draws.
+        const muted = host.activity_muted;
+        host.activity_muted = true;
+        defer host.activity_muted = muted;
         try editor.runActiveDocCommand(name);
+        var buf: [96]u8 = undefined;
+        const said = std.fmt.bufPrint(&buf, "Undid {s}'s {s}", .{ authorName(t.by), t.step.title }) catch "Undid a change";
+        if (editor.doc_surfaces.get(doc.id)) |d| d.feed.pushAction(said, "Redo", t.step.id, dvui.frameTimeNS());
     }
 }
 
+/// Redo `doc`'s step `id` from its bubble's Redo: only while it is still the next one to redo.
+pub fn redoFromFeed(editor: *Editor, doc: sdk.DocHandle, id: u64) void {
+    const t = editor.stepToTake(doc, .redo, editor.app.host.arena()) catch return;
+    if (t.step.id == id) _ = doc.owner.historyGoTo(doc, t.to);
+}
+
+pub const HistoryAction = enum { undo, redo };
+
+/// The step an undo or a redo of a document would take, the step it leaves on top (`to`, for
+/// `historyGoTo`; 0 before the first), and who made it.
+pub const StepToTake = struct { step: sdk.Plugin.HistoryLine.Entry, to: u64, by: sdk.Author };
+
+/// What an undo or a redo of `doc` would take, from its owner's line (`historyLine`):
+/// `error.NoLine` when the owner keeps none, `error.Nothing` when there is nothing to take.
+pub fn stepToTake(editor: *Editor, doc: sdk.DocHandle, action: HistoryAction, arena: std.mem.Allocator) error{ NoLine, Nothing }!StepToTake {
+    const line = doc.owner.historyLine(doc, arena) orelse return error.NoLine;
+    const step: sdk.Plugin.HistoryLine.Entry, const to: u64 = switch (action) {
+        .undo => blk: {
+            if (line.at == 0) return error.Nothing;
+            break :blk .{ line.entries[line.at - 1], if (line.at >= 2) line.entries[line.at - 2].id else 0 };
+        },
+        .redo => blk: {
+            if (line.at >= line.entries.len) return error.Nothing;
+            break :blk .{ line.entries[line.at], line.entries[line.at].id };
+        },
+    };
+    return .{ .step = step, .to = to, .by = editor.app.host.stepAuthor(doc.id, step.id) };
+}
+
+/// Who made a step, as a person reads it: "the person", else the author's label ("Claude").
+pub fn authorName(a: sdk.Author) []const u8 {
+    if (a.kind == .person) return "the person";
+    return if (a.label.len > 0) a.label else @tagName(a.kind);
+}
+
 pub fn openInFileBrowser(editor: *Editor, path: []const u8) !void {
     // Darwin goes through `darwin_spawn` rather than `std.process.run`: the latter walks the
     // `environ` array captured at startup, which any `unsetenv` elsewhere in the process (SDL,
 
@@ -442,40 +442,31 @@ fn cmdRedoWith(state: *anyopaque, args: HistoryStep.Args, call: *sdk.Command.Cal
 /// `historyGoTo`), so it reaches a document that is not active. With `expect`, only when that
 /// step is the one it would take, so a caller never takes back a step made since it looked
 /// (`plans/SHARED_HISTORY_PLAN.md`, step 5). Answers the step and whose it was.
-fn historyStepWith(editor: *Editor, args: HistoryStep.Args, call: *sdk.Command.Call, action: enum { undo, redo }) !void {
+fn historyStepWith(editor: *Editor, args: HistoryStep.Args, call: *sdk.Command.Call, action: Editor.HistoryAction) !void {
     const doc = if (args.path.len == 0)
         editor.activeDoc() orelse return call.fail("no document is active", .{})
     else
         editor.docFromPath(args.path) orelse return call.fail("no open document at {s}", .{args.path});
-    const line = doc.owner.historyLine(doc, call.arena) orelse {
+    const t = editor.stepToTake(doc, action, call.arena) catch |err| switch (err) {
+        error.Nothing => return call.fail("nothing to {s}", .{@tagName(action)}),
         // An owner with no line undoes the active document only, through its own command.
-        const active = editor.activeDoc();
-        if (args.expect != 0 or active == null or active.?.id != doc.id)
-            return call.fail("{s} keeps no history line, so it can only undo the active document, with no expect", .{doc.owner.id});
-        return switch (action) {
-            .undo => editor.undo(),
-            .redo => editor.redo(),
-        };
-    };
-    const host = &editor.app.host;
-    const step: sdk.Plugin.HistoryLine.Entry, const to: u64 = switch (action) {
-        .undo => blk: {
-            if (line.at == 0) return call.fail("nothing to undo", .{});
-            break :blk .{ line.entries[line.at - 1], if (line.at >= 2) line.entries[line.at - 2].id else 0 };
-        },
-        .redo => blk: {
-            if (line.at >= line.entries.len) return call.fail("nothing to redo", .{});
-            break :blk .{ line.entries[line.at], line.entries[line.at].id };
+        error.NoLine => {
+            const active = editor.activeDoc();
+            if (args.expect != 0 or active == null or active.?.id != doc.id)
+                return call.fail("{s} keeps no history line, so it can only {s} the active document, with no expect", .{ doc.owner.id, @tagName(action) });
+            return switch (action) {
+                .undo => editor.undo(),
+                .redo => editor.redo(),
+            };
         },
     };
-    const whose = host.stepAuthor(doc.id, step.id);
-    const by = if (whose.kind == .person) "the person" else if (whose.label.len > 0) whose.label else @tagName(whose.kind);
-    if (args.expect != 0 and args.expect != step.id) return call.fail(
+    const by = Editor.authorName(t.by);
+    if (args.expect != 0 and args.expect != t.step.id) return call.fail(
         "step {d} is not the one to {s}: that is step {d}, {s}, by {s}. The document changed since; fix it forward",
-        .{ args.expect, @tagName(action), step.id, step.title, by },
+        .{ args.expect, @tagName(action), t.step.id, t.step.title, by },
     );
-    if (!doc.owner.historyGoTo(doc, to)) return call.fail("{s} could not {s} step {d}", .{ doc.owner.id, @tagName(action), step.id });
-    try call.returns(.{ .step = step.id, .title = step.title, .by = by });
+    if (!doc.owner.historyGoTo(doc, t.to)) return call.fail("{s} could not {s} step {d}", .{ doc.owner.id, @tagName(action), t.step.id });
+    try call.returns(.{ .step = t.step.id, .title = t.step.title, .by = by });
 }
 /// Set while `tick` is running a command because of a live key event. That event is never
 /// marked handled, so it goes on to reach the focused widget under its own steam — a clipboard
 
@@ -112,6 +112,27 @@ fn canSaveAll(editor: *Editor) bool {
     return false;
 }
 
+/// "Undo Claude's Draw" when the step it would take is someone else's; "Undo" for the person's
+/// own, so working alone reads as it always has.
+fn undoTitle(editor: *Editor) [:0]const u8 {
+    return historyTitle(editor, .undo, "Undo", &undo_title);
+}
+fn redoTitle(editor: *Editor) [:0]const u8 {
+    return historyTitle(editor, .redo, "Redo", &redo_title);
+}
+
+var undo_title: [96:0]u8 = undefined;
+var redo_title: [96:0]u8 = undefined;
+
+fn historyTitle(editor: *Editor, action: Editor.HistoryAction, plain: [:0]const u8, buf: *[96:0]u8) [:0]const u8 {
+    const doc = editor.activeDoc() orelse return plain;
+    // Who made a step is kept only once someone besides the person has acted on the document.
+    if (!editor.app.host.step_authors.contains(doc.id)) return plain;
+    const t = editor.stepToTake(doc, action, editor.app.host.arena()) catch return plain;
+    if (t.by.kind == .person) return plain;
+    return std.fmt.bufPrintZ(buf, "{s} {s}'s {s}", .{ plain, Editor.authorName(t.by), t.step.title }) catch plain;
+}
+
 fn canUndo(editor: *Editor) bool {
     return editor.activeDocCommandEnabled("undo");
 }
@@ -193,8 +214,8 @@ const edit_items = [_]Item{
         .native_always_enabled = true,
     } },
     .separator,
-    .{ .command = .{ .id = "fizzy.undo", .title = .{ .static = "Undo" }, .sf_symbol = "arrow.uturn.backward", .enabled = canUndo } },
-    .{ .command = .{ .id = "fizzy.redo", .title = .{ .static = "Redo" }, .sf_symbol = "arrow.uturn.forward", .enabled = canRedo } },
+    .{ .command = .{ .id = "fizzy.undo", .title = .{ .dynamic = undoTitle }, .sf_symbol = "arrow.uturn.backward", .enabled = canUndo } },
+    .{ .command = .{ .id = "fizzy.redo", .title = .{ .dynamic = redoTitle }, .sf_symbol = "arrow.uturn.forward", .enabled = canRedo } },
     // Transform / Grid Layout are pixel-art concepts, not fizzy's own — pixi parents its own
     // section here rather than fizzy knowing about them.
     .{ .plugin_section = "fizzy.menu.edit" },
 
@@ -7533,6 +7533,32 @@ test "headless: the whole editor comes up, opens a file, plays the tour and goes
             try std.testing.expectEqualStrings(before, (try read(host, notes_args)).text);
         }
 
+        // The person's Cmd-Z taking a step someone else made: the menu names it, and the pane says
+        // so with a Redo that puts it back.
+        {
+            const front = editor.activeDoc() orelse return error.TestUnexpectedResult;
+            const before = (try read(host, "")).text;
+            const front_write = try std.fmt.allocPrint(gpa, ".{{ .path = \"{s}\", .text = \"// by Claude\\n\" }}", .{main_zig});
+            defer gpa.free(front_write);
+            const outer = host.act(.{ .kind = .agent, .plugin = "test", .session = 1, .label = "Claude" });
+            try std.testing.expect(host.callCommand("text.write", front_write, host.arena()) == .ok);
+            host.endAct(outer);
+            const taken = try editor.stepToTake(front, .undo, host.arena());
+            try std.testing.expectEqualStrings("Claude", fizzy.Editor.authorName(taken.by));
+
+            try editor.undo();
+            try std.testing.expectEqualStrings(before, (try read(host, "")).text);
+            const ds = editor.doc_surfaces.get(front.id) orelse return error.TestUnexpectedResult;
+            const said = ds.feed.items[ds.feed.len - 1];
+            try std.testing.expect(std.mem.startsWith(u8, said.title[0..said.len], "Undid Claude's "));
+            const redo = said.action orelse return error.TestUnexpectedResult;
+            try std.testing.expectEqual(taken.step.id, redo.tag);
+            editor.redoFromFeed(front, redo.tag);
+            try std.testing.expectEqualStrings("// by Claude\n", (try read(host, "")).text);
+            try editor.undo();
+            try std.testing.expectEqualStrings(before, (try read(host, "")).text);
+        }
+
         // Save and Close, the unsaved-changes dialog's: the same write Save makes, then the tab
         // closes once it lands.
         try std.testing.expect(doc.owner.isDirty(doc));