Only perform mutual recursion from the GUI thread

JUCE incorrectly calls `IComponentHandler::performEdit()` from the audio
thread instead of using the output parameter changes queue, so this
would also cause GUI resizes to be handled from the audio thread if they
come in at the same time as such a parameter change.

src/wine-host/bridges/vst3.h

@@ -294,8 +294,9 @@ class Vst3Bridge : public HostBridge {
     }
 
     /**
-     * Spawn a new thread and call `send_message()` from there, and then handle
-     * functions passed by calls to `do_mutual_recursion_on_gui_thread()` and
+     * When called form the GUI thread, spawn a new thread and call
+     * `send_message()` from there, and then handle functions passed by calls to
+     * `do_mutual_recursion_on_gui_thread()` and
      * `do_mutual_recursion_on_off_thread()` on this thread until we get a
      * response back. This is a very specific solution to a very specific
      * problem. When a plugin wants to resize itself, it will call
@@ -316,10 +317,24 @@ class Vst3Bridge : public HostBridge {
      * context.
      *
      * We apply the same trick in `Vst3HostBridge`.
+     *
+     * NOTE: This is meant to allow mutually recursive call chains where every
+     *       function is called from and handled on the GUI thread. JUCE calls
+     *       `IComponentHandler::performEdit` from the audio thread because they
+     *       didn't implement the VST3 output parameters, and if at the same
+     *       time a resize request comes in from the host that would mean that
+     *       the resize request is also called from the audio thread. To prevent
+     *       this we will make sure to only do this mutual recursion stuff if
+     *       this is actually called form the GUI thread.
      */
     template <typename T>
     typename T::Response send_mutually_recursive_message(const T& object) {
-        return mutual_recursion.fork([&]() { return send_message(object); });
+        if (main_context.is_gui_thread()) {
+            return mutual_recursion.fork(
+                [&]() { return send_message(object); });
+        } else {
+            return send_message(object);
+        }
     }
 
     /**