Flag-gated AppKit sidebar list (NSTableView rows + headers)

Resources/Localizable.xcstrings

@@ -100537,6 +100537,40 @@
         }
       }
     },
+    "featureFlags.appKitSidebarList.description": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Renders the workspace sidebar with a native AppKit list and divider for smoother scrolling and resizing with many workspaces."
+          }
+        },
+        "ja": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "ワークスペースサイドバーをネイティブの AppKit リストとディバイダーで描画し、多数のワークスペースでもスクロールとリサイズを滑らかにします。"
+          }
+        }
+      }
+    },
+    "featureFlags.appKitSidebarList.title": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Lawrence Sidebar"
+          }
+        },
+        "ja": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Lawrence サイドバー"
+          }
+        }
+      }
+    },
     "featureFlags.clearAll": {
       "extractionState": "manual",
       "localizations": {

Sources/App/WorkspaceRuntimeSettings.swift

@@ -12,7 +12,6 @@ enum WorkspaceTitlebarSettings {
 }
 enum WorkspacePresentationModeSettings {
     static let modeKey = "workspacePresentationMode"
-
     enum Mode: String {
         case standard
         case minimal

Sources/Debug/SidebarLazyContractProbe.swift

@@ -1,21 +1,18 @@
 import SwiftUI
 
 #if DEBUG
-/// Test-only body-evaluation probe for the workspace sidebar's lazy-layout
-/// contract: sidebar layout/diff work must stay O(visible rows), never
-/// O(all workspaces). The contract has regressed five times through five
-/// different mechanisms (#2586, #5764, #5845, #6210, #6556), each shipping to
-/// stable before being detected at scale. `SidebarLazyLayoutScaleTests` mounts
-/// the sidebar with hundreds of workspaces, injects these closures, and fails
-/// if row bodies are realized without bound or keep re-evaluating after
-/// updates settle — regardless of which mechanism defeats laziness next.
+/// Test-only probe for the workspace sidebar virtualization contract: AppKit
+/// must materialize viewport-many cells and reconfigure only changed hosted
+/// roots, never all workspaces. `SidebarLazyLayoutScaleTests` mounts hundreds
+/// of workspaces and fails on realization or reconfiguration churn.
 ///
 /// Same pattern as `MinimalModeInvalidationProbe`; compiled out of Release.
 struct SidebarLazyContractProbe {
     var workspaceRowBody: (() -> Void)?
     var workspaceRowBodyEnd: (() -> Void)?
     var groupHeaderRowBody: (() -> Void)?
     var workspaceSnapshotBuild: (() -> Void)?
+    var tableRootViewReconfigure: (() -> Void)?
     var workspaceRowInputProjection: (() -> Void)?
 }
 #endif

Sources/FeatureFlags.swift

@@ -47,6 +47,7 @@ final class CmuxFeatureFlags {
     private static let agentChatUIDefault = false
     private static let sidebarWorkspaceAgentSpinnerDefault = false
     private static let workspaceTodoControlsDefault = false
+    private static let appKitSidebarListDefault = false
 
     private static let overrideKeyPrefix = "cmux.flags.override."
 
@@ -151,6 +152,24 @@ final class CmuxFeatureFlags {
                 ),
                 defaultWhenUnavailable: CmuxFeatureFlags.workspaceTodoControlsDefault
             ),
+
+            // FLAG(key: sidebar-appkit-list-experiment, owner: lawrencecchen,
+            //      reviewBy: 2026-10-01, defaultWhenUnavailable: false)
+            // Renders the workspace sidebar with the AppKit NSTableView list
+            // (virtualized rows, measured-once heights) instead of the SwiftUI
+            // LazyVStack. Off by default while the rewrite soaks.
+            CmuxFeatureFlagDefinition(
+                key: "sidebar-appkit-list-experiment",
+                title: String(
+                    localized: "featureFlags.appKitSidebarList.title",
+                    defaultValue: "Lawrence Sidebar"
+                ),
+                flagDescription: String(
+                    localized: "featureFlags.appKitSidebarList.description",
+                    defaultValue: "Renders the workspace sidebar with a native AppKit list and divider for smoother scrolling and resizing with many workspaces."
+                ),
+                defaultWhenUnavailable: CmuxFeatureFlags.appKitSidebarListDefault
+            ),
         ]
     }()
 
@@ -178,6 +197,10 @@ final class CmuxFeatureFlags {
         effectiveValue(for: Self.allFlags[5])
     }
 
+    var isAppKitSidebarListEnabled: Bool {
+        effectiveValue(for: Self.allFlags[6])
+    }
+
     @ObservationIgnored
     private let defaults: UserDefaults
     @ObservationIgnored

Sources/GhosttyTerminalView.swift

@@ -9113,13 +9113,22 @@ final class GhosttySurfaceScrollView: NSView {
     }
 
     private func logLayoutDuringActiveDrag(targetSize: CGSize) {
+        // Cheap app-owned gates first. The NSPasteboard(name: .drag) read
+        // below is a synchronous XPC round trip to the pasteboard daemon;
+        // unguarded it ran on every pane layout of every width commit during
+        // sidebar divider drags (5% of main-thread time in a Time Profiler
+        // capture) because divider drags also deliver drag mouse events.
+        let dropZoneActive = activeDropZone != nil || pendingDropZone != nil
+        let appOwnedDragActive =
+            AppDelegate.shared?.sidebarWorkspaceDragRegistry.currentWorkspaceId != nil
+        guard dropZoneActive || appOwnedDragActive else { return }
+
         let pasteboardTypes = NSPasteboard(name: .drag).types
         let hasTabDrag = pasteboardTypes?.contains(Self.tabTransferPasteboardType) == true
         let hasSidebarDrag = pasteboardTypes?.contains(Self.sidebarTabReorderPasteboardType) == true
         let eventType = NSApp.currentEvent?.type
         let hasActiveDrag =
-            activeDropZone != nil ||
-            pendingDropZone != nil ||
+            dropZoneActive ||
             ((hasTabDrag || hasSidebarDrag) && Self.isDragMouseEvent(eventType))
         guard hasActiveDrag else { return }
 

Sources/Sidebar/AppKitList/Cells/SidebarGroupHeaderRowModel.swift

@@ -0,0 +1,58 @@
+import CoreGraphics
+import Foundation
+
+/// Immutable render input for one pure-AppKit sidebar group header row.
+///
+/// Value fields only: action closures live in ``SidebarGroupHeaderRowActions``
+/// and are excluded from equality so recycled cells can reconfigure cheaply
+/// (same discipline as the hosted rows' Equatable snapshot contract).
+struct SidebarGroupHeaderRowModel: Equatable {
+    let groupId: UUID
+    let anchorWorkspaceId: UUID
+    let name: String
+    let iconSymbol: String
+    let tintHex: String?
+    let isCollapsed: Bool
+    let isPinned: Bool
+    let isAnchorActive: Bool
+    let memberCount: Int
+    let anchorUnreadCount: Int
+    let canMarkRead: Bool
+    let canMarkUnread: Bool
+    let hasLatestNotifications: Bool
+    let canMarkAllRead: Bool
+    let canMarkAllUnread: Bool
+    /// Resolved modifier-hold hint (for example "⌘3"); nil hides the pill.
+    let shortcutHintText: String?
+    let shortcutHintXOffset: Double
+    let shortcutHintYOffset: Double
+    let fontScale: CGFloat
+    let globalFontMagnificationPercent: Int
+    let cwdContextMenuItems: [CmuxResolvedConfigContextMenuItem]
+    let rowSpacing: CGFloat
+    let isFirstRow: Bool
+    let isBeingDragged: Bool
+    let topDropIndicatorVisible: Bool
+    let bottomDropIndicatorVisible: Bool
+}
+
+/// Behavior bundle for one group header row; recreated per apply and excluded
+/// from model equality.
+@MainActor
+struct SidebarGroupHeaderRowActions {
+    let onToggleCollapsed: () -> Void
+    let onFocusAnchor: () -> Void
+    let onTapPlus: () -> Void
+    let onRunResolvedItem: (CmuxResolvedConfigMenuAction) -> Void
+    let onRename: () -> Void
+    let onTogglePinned: () -> Void
+    let onMarkRead: () -> Void
+    let onMarkUnread: () -> Void
+    let onClearLatestNotifications: () -> Void
+    let onMarkAllRead: () -> Void
+    let onMarkAllUnread: () -> Void
+    let onUngroup: () -> Void
+    let onDelete: () -> Void
+    let onEditConfig: () -> Void
+    let onOpenDocs: () -> Void
+}