Fix double-click to zoom in sidebar title bar area

Sources/ContentView.swift

@@ -579,6 +579,12 @@ struct VerticalTabsSidebar: View {
                     SidebarTopScrim(height: trafficLightPadding + 20)
                         .allowsHitTesting(false)
                 }
+                .overlay(alignment: .top) {
+                    // Double-click the sidebar title-bar area to zoom the
+                    // window, matching the panel top-bar behaviour.
+                    DoubleClickZoomView()
+                        .frame(height: trafficLightPadding)
+                }
                 .background(Color.clear)
                 .modifier(ClearScrollBackground())
             }
@@ -1920,6 +1926,28 @@ private struct SidebarTabDropDelegate: DropDelegate {
     }
 }
 
+/// AppKit-level double-click handler for the sidebar title-bar area.
+/// Uses NSView hit-testing so it isn't swallowed by the SwiftUI ScrollView underneath.
+private struct DoubleClickZoomView: NSViewRepresentable {
+    func makeNSView(context: Context) -> NSView {
+        DoubleClickZoomNSView()
+    }
+
+    func updateNSView(_ nsView: NSView, context: Context) {}
+
+    private final class DoubleClickZoomNSView: NSView {
+        override var mouseDownCanMoveWindow: Bool { true }
+        override func hitTest(_ point: NSPoint) -> NSView? { self }
+        override func mouseDown(with event: NSEvent) {
+            if event.clickCount == 2 {
+                window?.zoom(nil)
+            } else {
+                super.mouseDown(with: event)
+            }
+        }
+    }
+}
+
 private struct MiddleClickCapture: NSViewRepresentable {
     let onMiddleClick: () -> Void