Sets the toplevel to be modal.
The application can use this hint to tell the window manager that a certain surface has modal behaviour. The window manager can use this information to handle modal surfaces in a special way.
You should only use this on surfaces for which you have previously called [[email protected]_transient_for].
| this |
a `GdkToplevel` |
| modal |
true if the surface is modal, false otherwise. |