1) Build Qt with Glib support enabled.
2) Run qtbase/examples/widgets/richtext/textedit with QT_NO_GLIB=1 exported.

QT_NO_GLIB forces Qt to use Unix event dispatcher.

3) From menu select "Format" -> "Color...". This will open native GTK dialog, by calling QGtk3Dialog::exec() -> gtk_dialog_run() which will spin a local Glib native event loop, but since Qt application is running with Unix dispatcher, Qt won't handle any events until the dialog is dismissed. For example: try resizing main window while the dialog is open, it won't repaint.

Any GTK-based code in Qt should first check for QT_NO_GLIB when selecting which code path to run - native or qt.