A card window toolbar control shows and hides the trailing sidebar — View ▸ Show Sidebar joins Edit Body, Raw Source and Add Attachment

The card window's trailing attributes sidebar has always been unconditional
since m6; this adds View ▸ Show Sidebar (a checkmark toggle, ShowComments'
shape) and a matching toolbar item on the customizable card toolbar
(NSToolbar/WindowToolbarController), a fourth default beside Edit Body,
Raw Source and Add Attachment. sidebar.right for the trailing pane; one
shared animated write path (AppPreferences.setShowCardSidebar) both faces
call, structural-voice reflow with a trailing slide-and-fade transition
(Motion.cardSidebarTransition), Reduce Motion respected throughout.

CardWindowMetrics.minimumSize gains a sidebar: Bool = true parameter so a
hidden sidebar shrinks the window's floor, composing with the existing
commentsColumn parameter. The toolbar item's read/write are injectable
closures (defaulted to the real UserDefaults-backed pair) so its plumbing
is testable without touching the developer's own preferences domain.
WindowToolbarController's observation tracking only sees @Observable
reads, so a small HostedWindowController.revalidateToolbar() plus an
onChange nudge keeps the toolbar button's on-state in step with the
View-menu row's write.

Scope held narrowly to visibility, per the card: no sidebar section
reordering, no action-moving.

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

KanbanTests/ToolbarTests.swift

@@ -598,16 +598,16 @@ struct CardToolbarTests {
         return (body, raw, attachments)
     }
 
-    @Test("The default set is the whole catalog — the trio, in 03's order")
+    @Test("The default set is the whole catalog — the trio plus Show Sidebar, in this file's order")
     func defaultsAreTheCatalog() {
         let (body, raw, attachments) = makeHandles()
         let specs = CardToolbar.specs(body: body, rawSource: raw, attachments: attachments)
 
         // "Card window default: Edit Body · Raw Source · Add Attachment … the catalog is the same
-        // trio."
-        #expect(CardToolbar.defaultItems == [.cardEditBody, .cardRawSource, .cardAddAttachment])
+        // trio" — joined by Show Sidebar, the fourth default item the toolbar-toggle card added.
+        #expect(CardToolbar.defaultItems == [.cardEditBody, .cardRawSource, .cardAddAttachment, .cardShowSidebar])
         #expect(specs.map(\.identifier) == CardToolbar.defaultItems)
-        #expect(specs.map(\.label) == ["Edit Body", "Raw Source", "Add Attachment"])
+        #expect(specs.map(\.label) == ["Edit Body", "Raw Source", "Add Attachment", "Show Sidebar"])
     }
 
     @Test("Every item's symbol resolves on this system")
@@ -711,6 +711,45 @@ struct CardToolbarTests {
         #expect(!addAttachment.isEnabled)
         #expect(addAttachment.isEnabled == AddAttachmentCommand.isEnabled(attachments))
     }
+
+    /// **Show Sidebar is always enabled and drives its own injected write path** — the two closures
+    /// `CardToolbar.specs` defaults to `AppPreferences.showCardSidebar` / `.setShowCardSidebar`, held
+    /// here over a local `Bool` instead so this test never touches the developer's own
+    /// `UserDefaults.standard` domain (`CardSessionUndoTests`' quick-style caution, applied to the one
+    /// item here with no window-scoped handle to hold a scratch value instead).
+    @Test("Show Sidebar toggles its own bit, always enabled, on-state matching the read")
+    func showSidebarTogglesItsOwnBit() {
+        let (body, raw, attachments) = makeHandles()
+        var shown = true
+        let specs = CardToolbar.specs(
+            body: body,
+            rawSource: raw,
+            attachments: attachments,
+            isSidebarShown: { shown },
+            setSidebarShown: { shown = $0 }
+        )
+        guard let showSidebar = specs.spec(.cardShowSidebar) else {
+            Issue.record("no Show Sidebar item")
+            return
+        }
+
+        #expect(showSidebar.isOn == true)
+        #expect(showSidebar.isEnabled, "showing or hiding a pane is not a mutation the read-only lock gates")
+
+        showSidebar.activate()
+        #expect(shown == false, "the item drives the same bit the injected closures read")
+        #expect(showSidebar.isOn == false)
+
+        showSidebar.activate()
+        #expect(shown == true)
+        #expect(showSidebar.isOn == true)
+
+        // Raw source and the read-only lock are both the *other* items' predicates — Show Sidebar
+        // reads neither.
+        raw.enter()
+        attachments.isEditable = false
+        #expect(showSidebar.isEnabled)
+    }
 }
 
 // MARK: - ⌘F and the search field's two homes