Still add support for multiple context menus

Even though having more than one of these open at a time will be
impossible, I could see some misbehaving plugin keeping an old context
menu pointer around and dropping it when another context menu is open,
which would otherwise cause the new context menu to get destroyed.

src/common/serialization/vst3/context-menu-proxy.h

@@ -34,12 +34,6 @@
  * the user clicks on them. As far as I'm aware, not a single Linux VST3 host
  * implements `IComponentHandler3` and thus provides support for these context
  * menus.
- *
- * NOTE: For simplicity's sake (and because this is going to be true 100% of the
- *       time) we'll assume a plugin can only have a single context menu open at
- *       a time. If a host does allow creating multiple context menus for
- *       different parameters at the same time, then we'll just have to add a
- *       unique instance ID to the context menu.
  */
 class Vst3ContextMenuProxy : public YaContextMenu {
    public:
@@ -61,6 +55,14 @@ class Vst3ContextMenuProxy : public YaContextMenu {
          * context menu has been created for.
          */
         native_size_t owner_instance_id;
+        /**
+         * A unique identifier for this specific context menu. Having more than
+         * one context menu at a time will be impossible, but in case the plugin
+         * for whatever reason hangs on to the pointer of an old context menu
+         * after it has opened a new one, we would not want the new context menu
+         * to get destroyed when it drops the old pointer.
+         */
+        native_size_t context_menu_id;
 
         YaContextMenu::ConstructArgs context_menu_args;
 
@@ -80,6 +82,11 @@ class Vst3ContextMenuProxy : public YaContextMenu {
      * message. When the object's reference count reaches zero, we should
      * destroy the actual context menu object provided by the host using the
      * `Destruct` message.
+     *
+     * @note The lifecycle of these objects should be tracked in an
+     *   `std::map<size_t, Vst3ContextMenuProxy*>` in the `InstanceInterfaces`
+     *   struct. We need to use raw pointers or references here so we can refer
+     *   to the object without interfering with the reference count.
      */
     Vst3ContextMenuProxy(const ConstructArgs&& args);
 
@@ -92,10 +99,12 @@ class Vst3ContextMenuProxy : public YaContextMenu {
         using Response = Ack;
 
         native_size_t owner_instance_id;
+        native_size_t context_menu_id;
 
         template <typename S>
         void serialize(S& s) {
             s.value8b(owner_instance_id);
+            s.value8b(context_menu_id);
         }
     };
 
@@ -115,6 +124,11 @@ class Vst3ContextMenuProxy : public YaContextMenu {
         return arguments.owner_instance_id;
     }
 
+    /**
+     * Get the unique ID for this context menu
+     */
+    inline size_t context_menu_id() const { return arguments.context_menu_id; }
+
    private:
     ConstructArgs arguments;
 };