Another (IMHO false) fall-through warning silenced

With a FIXME. The case should be checked.

src/frontends/qt4/GuiApplication.cpp

@@ -2723,7 +2723,8 @@ bool GuiApplication::notify(QObject * receiver, QEvent * event)
 #endif
 			// In release mode, try to exit gracefully.
 			this->exit(1);
-
+			// FIXME: GCC 7 thinks we can fall through here. Can we?
+			// fall through
 		case BufferException: {
 			if (!current_view_ || !current_view_->documentBufferView())
 				return false;