restrict performKeyEquivalent to editor-handled shortcuts only, restore system menu items

src/IcedViewportView.swift

@@ -150,6 +150,33 @@ class IcedViewportView: NSView {
 
     // MARK: - Key Events
 
+    override func performKeyEquivalent(with event: NSEvent) -> Bool {
+        guard viewportHandle != nil else { return super.performKeyEquivalent(with: event) }
+        let flags = event.modifierFlags.intersection(.deviceIndependentFlagsMask)
+        let cmd = flags.contains(.command)
+        let shift = flags.contains(.shift)
+        let chars = event.charactersIgnoringModifiers ?? ""
+
+        if cmd && !shift {
+            switch chars {
+            case "a", "c", "v", "x", "z", "p",
+                 "=", "+", "-", "0":
+                keyDown(with: event)
+                return true
+            default: break
+            }
+        }
+        if cmd && shift {
+            switch chars {
+            case "z":
+                keyDown(with: event)
+                return true
+            default: break
+            }
+        }
+        return super.performKeyEquivalent(with: event)
+    }
+
     override func keyDown(with event: NSEvent) {
         guard let h = viewportHandle else { return }
         let text = event.characters ?? ""