macos: invalid restorable state whenever surface tree changes

Related to #1177

This detects whenever the surface tree (splits) change in any way and
requests that the restorable state be invalided by macOS.

macos/Sources/Features/Terminal/TerminalController.swift

@@ -470,6 +470,12 @@ class TerminalController: NSWindowController, NSWindowDelegate,
         self.window?.close()
     }
     
+    func surfaceTreeDidChange() {
+        // Whenever our surface tree changes in any way (new split, close split, etc.)
+        // we want to invalidate our state.
+        invalidateRestorableState()
+    }
+    
     //MARK: - Clipboard Confirmation
     
     func clipboardConfirmationComplete(_ action: ClipboardConfirmationView.Action, _ request: Ghostty.ClipboardRequest) {

macos/Sources/Features/Terminal/TerminalView.swift

@@ -13,6 +13,10 @@ protocol TerminalViewDelegate: AnyObject {
     
     /// The cell size changed.
     func cellSizeDidChange(to: NSSize)
+    
+    /// The surface tree did change in some way, i.e. a split was added, removed, etc. This is
+    /// not called initially.
+    func surfaceTreeDidChange()
 }
 
 // Default all the functions so they're optional
@@ -97,6 +101,12 @@ struct TerminalView<ViewModel: TerminalViewModel>: View {
                         guard let size = newValue else { return }
                         self.delegate?.cellSizeDidChange(to: size)
                     }
+                    .onChange(of: viewModel.surfaceTree?.hashValue) { _ in
+                        // This is funky, but its the best way I could think of to detect
+                        // ANY CHANGE within the deeply nested surface tree -- detecting a change
+                        // in the hash value.
+                        self.delegate?.surfaceTreeDidChange()
+                    }
             }
         }
     }