Diagnosed by sampling a live frozen instance: with a text surface focused,
RedoMenuRow's body reads the routed manager's title, NSUndoManager.canRedo
posts NSUndoManagerCheckpoint synchronously, UndoCommandTicker bumps its
observed revision mid-body, and SwiftUI schedules the re-evaluation whose
own read posts the next checkpoint — the main thread never returns to the
event loop (~99% CPU, app frozen). Board-routed reads never echo, because
BoardUndoManager's overrides answer from the provider without posting —
which is why the board-only live probe (21/21) never met the loop.
The rows now derive title and enablement inside
UndoCommandTicker.silencingReadEchoes, a synchronous main-actor window in
which bump() drops what arrives: a read cannot change the state it reads,
so the echo carries no information and dropping it loses nothing. Genuine
checkpoints — registration closing a group, a crossing — still land.
Three regression tests pin the mechanism, including the asymmetry that
made the redo side the fuel: canUndo answers silently, canRedo posts.
Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY
Edit ▸ Undo/Redo become the app's own replaced rows and the board toolbar
pair takes explicit targets, both reading the focused session's
BoardUndoManager through FocusedValues.undoStack (board windows publish the
session's manager, card windows their own) — the nil-target route died with
the SwiftUI window latch, 13-native-undo.md ▸ Rules ▸ command surface,
re-ruled 2026-08-08. The rows enact the routing predicate themselves: text
focus routes ⌘Z to the first responder's own manager, title and enablement
included, re-derived at fire time with a beep for the stale window.
NativeHistoryProvider turns @Observable so both surfaces re-derive on stack
changes; a checkpoint-notification ticker covers plain text managers.
.responderAction leaves ToolbarItemSpec with its only user;
windowWillReturnUndoManager stays wired for AppKit's own asks.
Live-probed on the fixture board (21/21): the row retitles to "Undo Add
Lane" and crosses via real ⌘Z key events, ⇧⌘Z redoes via a window-server
chord, the toolbar pair validates and fires, search-field and body-editor
⌘Z stay text undo with board stacks untouched, and a card window crosses
its own stack with no fall-through. 2698 unit tests green.
Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy
rzen