The way gdk_drag_status() may be called multiple times during the
processing of drag and drop events throughout the widget hierarchy
brings some superfluous messaging going in, esp. when it's the last
request the one we want to honor, yet we emit messaging requests on
all.
This is barely appreciable in the X11 backend, but due to the design
of the wayland protocol, quick series of changes like this it have
some self-amplificating consequences which may end up flooding the
connection.
We can delegate this to a late "commit" call, performed within GDK
event management. This way gdk_drag_status() calls may be cached
and only result in windowing messaging once per ::drag-motion or
::drag-data-received event. Emitting the final status will also
avoid spurious action changes on the compositor and the other peer.
https://bugzilla.gnome.org/show_bug.cgi?id=763298
That way we can let ::cancel callers to override the visual
result of the operation (eg. when detaching notebook tabs on
NO_TARGET).
Also, document gdk_drag_drop_done() so it is mentioned that
this is a one-shot call.
https://bugzilla.gnome.org/show_bug.cgi?id=761954
We should conform to a minimal set of reasons for the gtk side to emit
a better GtkDragResult than GTK_DRAG_RESULT_ERROR. This fixes the notebook
tab DnD feature, where we rely on GTK_DRAG_RESULT_NO_TARGET.
In the wayland side, unfortunately we can't honor either NO_TARGET nor
USER_CANCELLED, we don't know of the latter, so we could return false
positives on the former.
https://bugzilla.gnome.org/show_bug.cgi?id=761954
This function (most similar to gtk_drag_get_cursor() helps figure out
the right cursor that applies to a given action. To be used by the
various backends.
We've traditionally left GTK+ to handle the input side of things,
letting GDK only manage the windowing-specific messaging. This
way of splitting responsibilities is not compatible however with
some backends, we must fold then input management at the DnD stage
into GDK (and backends) domain.
The gdk_drag_context_manage_dnd() call is meant to be the entry
point for this mode of operation, if the drag and drop operation
becomes managed, the caller (i.e. gtkdnd.c) doesn't need to perform
grabs, nor manage input events itself.
As a consequence of this, different aspects now belong to the
backend GdkDragContext implementation:
- Because the caller doesn't see keyboard events anymore,
keyboard navigation must be managed in GDK, so is the decision
of the current action based on modifiers/button pressed.
- Because the caller won't see input events in general, the lifetime
of the drag and drop operation is now communicated through the
::drop-performed, ::dnd-finished and ::cancel events
- Because the caller doesn't participate anymore on the action
being chosen, the pointer cursor must be set by the backend.
The caller is rather notified of the final action through the
::action signal.
The caller is still responsible of dealing with the corresponding
GdkSelection, ensuring its ownership and communicating the supported
mimetypes.
