GDK has a lock to mark critical sections inside the backends.
Additionally, code that would re-enter into the GTK main loop was
supposed to hold the lock.
Back in the Good Old Days™ this was guaranteed to kind of work only on
the X11 backend, and would cause a neat explosion on any other GDK
backend.
During GTK+ 3.x we deprecated the API to enter and leave the critical
sections, and now we can remove all the internal uses of the lock, since
external API that uses GTK+ 4.x won't be able to hold the GDK lock.
https://bugzilla.gnome.org/show_bug.cgi?id=793124
The main GDK thread lock is not portable and deprecated.
The only reason why gdk_threads_add_idle() and
gdk_threads_add_idle_full() exist is to allow invoking a callback with
the GDK lock held, in case 3rd party libraries still use the deprecated
gdk_threads_enter()/gdk_threads_leave() API.
Since we're removing the GDK lock, and we're releasing a new major API,
such code cannot exist any more; this means we can use the GLib API for
installing idle callbacks.
https://bugzilla.gnome.org/show_bug.cgi?id=793124
The query function for cursor sizes and capabilities
are not very interesting. At least, they are not used
in GTK+, and all backends but X11 just hardcode
made-up values anyway. So, lets drop them.
Without selections, drags can't have them either.
Also included is removing the selection from GtkSelectionData.
Includes a bunch of crude cleanups to Wayland code that no longer has to
care about selection atoms.
Instead of just passing the GdkContentFormats, we are now passing the
GdkContentProvider to gdk_drag_begin().
This means that GDK itself can now query the data from the provider
directly instead of having to send selection events.
Use this to provide the private API gdk_drag_context_write() that allows
backends to pass an output stream that this data will be written to.
Implement this as the mechanism for providing drag data on Wayland.
And to make this all work, implement a content provider named
GtkDragContent that is implemented by reverting to the old DND
drag-data-get machinery inside GTK, so for widgets everything works just
like before.
Instead, pass the actions as part of gdk_drag_begin() and insist DND is
always managed.
A new side effect is that gdk_drag_begin() can now return %NULL.
