Once a window has been marked as unresizable, it cannot be resized again. Maximizing the window still works as expected. For example, when activating and deactivating parole's new mini mode (git master), the resizable flag is toggled off and on. gtk_window_set_resizable (window, FALSE); gtk_window_set_resizable (window, TRUE); The behavior can be observed in the below video. https://youtu.be/n0Q1i5PSKiU
I observe the same with programs using Java and JavaFX. In a larger project I open modal dialogs using instances of Stage (JavaFX) and assign it the reference of the owner window: Stage stage = new Stage(); stage.initOwner(ownerWindow); // where the 'owner window' is running in an XFCE window. After this step the XFCE window can no longer be resized. It shows the same behaviour as reported in this bug. The respective menu item in the XFCE window menu (upper left corner) is shaded grey to indicate its disabled status.