diff --git a/libs/ardour/session_vst.cc b/libs/ardour/session_vst.cc
index 3252698bf4..2720334ad5 100644
--- a/libs/ardour/session_vst.cc
+++ b/libs/ardour/session_vst.cc
@@ -496,6 +496,10 @@ intptr_t Session::vst_callback (
 		if (session) {
 			session->set_dirty ();
 		}
+		/* allow plugin UI to update */
+#ifdef WINDOWS_VST_SUPPORT
+		fst_audio_master_idle();
+#endif
 		return 0;
 
 	case audioMasterBeginEdit: