The titlebar stand-in stops reappearing on a window that attaches after the board has loaded

`configureWindow` removed the loading stand-in from whatever window it happened to be
running against, but left `onAttach` installed exactly as `configureLoadingWindow` set
it — a closure that builds a fresh stand-in on every attach, deliberately, so it survives
the provisional-window swap while the board is still loading. SwiftUI's dismantle-then-
make swap has no deadline, though: when it lands after the store has already loaded, the
stale closure reinstalls a stand-in on the real window a moment before `attach`'s own
`addTitlebarAccessoryIfPossible()` installs the real widget beside it — both in the
titlebar, stand-in leading, and nothing left holding a reference to remove it a second
time. `onAttach` is now replaced in `configureWindow` too, keeping only the frame-
placement half a re-attached window still needs. New suite
`BoardLoadingTitlebarSwapRaceTests` drives `HostedWindowController` through the same
attach sequence and pins both the fixed and (as a negative control) the unfixed shape.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

KanbanTests/BoardLoadingTests.swift

@@ -243,3 +243,121 @@ struct BoardLoadingTitlebarStandInTests {
         #expect(window.titlebarAccessoryViewControllers.count == 1, "nothing to remove, nothing removed")
     }
 }
+
+// MARK: - The reopened bug: a late provisional-to-real swap
+
+/// **Pipeline card a73bad86, reopened 2026-08-09**: the removal above is not the whole fix.
+/// `configureWindow` removed the stand-in from *the window it happened to run against*, but left
+/// `windowController.onAttach` exactly as `configureLoadingWindow` had installed it — a closure that
+/// builds a *fresh* stand-in and adds it to whatever window attaches next, deliberately, so it
+/// survives the provisional-window swap while the board is still loading
+/// (`HostedWindowController.detach`'s own doc comment: "install arrives before any window, a
+/// dismantle follows, and only then does the real window attach").
+///
+/// The trouble is that swap has no deadline. When it lands *after* the store has already loaded and
+/// `configureWindow` has already run, the stale closure is still the one `HostedWindowController
+/// .attach` calls — it reinstalls a stand-in on the real window a moment before `attach`'s own
+/// `addTitlebarAccessoryIfPossible()` installs the real widget beside it. Both sit in the titlebar,
+/// stand-in leading — exactly the owner's screenshot — and nothing left with a reference to the
+/// stand-in ever removes it. "Occasionally" was this ordering: whenever the load wins the race
+/// against the swap.
+///
+/// `BoardWindowHost` is a SwiftUI view struct with no seam of its own to call `configureWindow`
+/// directly, so this drives `HostedWindowController` through the same sequence that method does —
+/// `configureLoadingWindow`'s attach, then the fixed `configureWindow`'s replacement of both the
+/// stand-in and `onAttach` itself, then a late attach standing in for the delayed swap — the same
+/// structural level `BoardLoadingTitlebarStandInTests` above tests at.
+@MainActor
+@Suite("Board loading state ▸ the provisional-window swap race")
+struct BoardLoadingTitlebarSwapRaceTests {
+
+    private static func window() -> NSWindow {
+        NSWindow(
+            contentRect: NSRect(x: 0, y: 0, width: 600, height: 400),
+            styleMask: [.titled, .closable, .miniaturizable, .resizable],
+            backing: .buffered,
+            defer: true
+        )
+    }
+
+    @Test("A late provisional-to-real attach installs only the real widget, never a second stand-in")
+    func lateAttachAfterLoadDoesNotReinstallTheStandIn() {
+        let controller = HostedWindowController()
+        var loadingAccessory: NSTitlebarAccessoryViewController?
+
+        // `configureLoadingWindow`'s own closure, verbatim: a fresh stand-in per attach, held so a
+        // later removal can find it by identity.
+        controller.onAttach = { window in
+            let accessory = boardLoadingTitlebarAccessory(title: "Roadmap")
+            loadingAccessory = accessory
+            window.addTitlebarAccessoryViewController(accessory)
+        }
+
+        // The loading window the user sees first — the provisional window's attach.
+        let provisional = Self.window()
+        controller.attach(to: provisional)
+        #expect(provisional.titlebarAccessoryViewControllers.count == 1, "the stand-in, and only the stand-in")
+
+        // The load finishes before the real-window swap arrives: `configureWindow`'s own sequence,
+        // this suite's fix included — removal by identity, then `onAttach` replaced rather than left
+        // standing.
+        let realWidget = boardLoadingTitlebarAccessory(title: "Roadmap")
+        if let loadingAccessory,
+           let index = provisional.titlebarAccessoryViewControllers.firstIndex(where: { $0 === loadingAccessory }) {
+            provisional.removeTitlebarAccessoryViewController(at: index)
+        }
+        loadingAccessory = nil
+        controller.installTitlebarAccessory(realWidget)
+        // The fix under test: `onAttach` keeps only the frame-placement half a re-attached window
+        // still needs — no stand-in built or installed for anything that attaches from here on.
+        controller.onAttach = { _ in }
+
+        #expect(provisional.titlebarAccessoryViewControllers.count == 1, "the real widget replaced the stand-in")
+        #expect(provisional.titlebarAccessoryViewControllers.first === realWidget)
+
+        // SwiftUI's dismantle-then-make swap, landing *after* the load — the exact ordering the
+        // reopened bug depended on. `detach()` releases the provisional window without discarding the
+        // held chrome; the real window's `attach` reinstalls it.
+        controller.detach()
+        let real = Self.window()
+        controller.attach(to: real)
+
+        #expect(
+            real.titlebarAccessoryViewControllers.count == 1,
+            "no stand-in reappears on a window that attaches after the board has already loaded"
+        )
+        #expect(real.titlebarAccessoryViewControllers.first === realWidget, "only the real widget carries over")
+    }
+
+    @Test("Without the fix, the same late attach reinstalls a stand-in beside the real widget")
+    func theUnfixedClosureReproducesTheDuplicate() {
+        // The regression's own negative space: this pins that the scenario above is not vacuously
+        // true by construction — with `configureWindow` never replacing `onAttach` (the bug this
+        // card reopened over), the same late swap really does leave two accessories installed.
+        let controller = HostedWindowController()
+        var loadingAccessory: NSTitlebarAccessoryViewController?
+        controller.onAttach = { window in
+            let accessory = boardLoadingTitlebarAccessory(title: "Roadmap")
+            loadingAccessory = accessory
+            window.addTitlebarAccessoryViewController(accessory)
+        }
+
+        let provisional = Self.window()
+        controller.attach(to: provisional)
+
+        let realWidget = boardLoadingTitlebarAccessory(title: "Roadmap")
+        if let loadingAccessory,
+           let index = provisional.titlebarAccessoryViewControllers.firstIndex(where: { $0 === loadingAccessory }) {
+            provisional.removeTitlebarAccessoryViewController(at: index)
+        }
+        loadingAccessory = nil
+        controller.installTitlebarAccessory(realWidget)
+        // `onAttach` deliberately left untouched — the pre-fix state.
+
+        controller.detach()
+        let real = Self.window()
+        controller.attach(to: real)
+
+        #expect(real.titlebarAccessoryViewControllers.count == 2, "the bug: a fresh stand-in beside the real widget")
+    }
+}