xdg-shell: Add support for explicit popup repositioning

This commit adds protocol additions making it possible to request that a
popup should be repositioned according to a new xdg_positioner object.

Explicit popup moving is done using a new request on xdg_popup:
xdg_popup.reposition. What it does is change the parameters used for
positioning a popup by providing a new xdg_positioner object. This
request is coupled with a new event; xdg_popup.repositioned, sent
together with the configure events (xdg_popup.configure and
xdg_surface.configure) to notify about the completion of the reposition
request. The reposition request also takes a token that is later passed
via the repositioned event; this is done so that a client may determine
for which reposition request the compositor has sent configure events.

Synchronization between surfaces to avoid state application race
condition are deliberately left out, and should be handled by an
external protocol.

To brief the compositor of the future dimension of the parent that the
compositor should position the popup against, a
xdg_positioner.set_parent_size request is added.

Lastly, a request to couple a xdg_positioner object with a parent
configure event is added (xdg_positioner.set_parent_configure) in
order for a compositor to pair a popup reposition request with a pending
configure event, and it's resulting window geometry. This is necessary
to, for example, properly constrain a popup given a future parent state.
An example of when this may be necessary is an interactive resize where
both the toplevel position and the relative popup position changes.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>

stable/xdg-shell/xdg-shell.xml

@@ -370,6 +370,33 @@
 	xdg_surface.configure event.
       </description>
     </request>
+
+    <request name="set_parent_size" since="3">
+      <description summary="">
+	Set the parent window geometry the compositor should use when
+	positioning the popup. The compositor may use this information to
+	determine the future state the popup should be constrained using. If
+	this doesn't match the dimension of the parent the popup is eventually
+	positioned against, the behavior is undefined.
+
+	The arguments are given in the surface-local coordinate space.
+      </description>
+      <arg name="parent_width" type="int"
+	   summary="future window geometry width of parent"/>
+      <arg name="parent_height" type="int"
+	   summary="future window geometry height of parent"/>
+    </request>
+
+    <request name="set_parent_configure" since="3">
+      <description summary="set parent configure this is a response to">
+	Set the serial of a xdg_surface.configure event this positioner will be
+	used in response to. The compositor may use this information together
+	with set_parent_size to determine what future state the popup should be
+	constrained using.
+      </description>
+      <arg name="serial" type="uint"
+	   summary="serial of parent configure event"/>
+    </request>
   </interface>
 
   <interface name="xdg_surface" version="3">
@@ -1138,7 +1165,7 @@
 	For version 2 or older, the configure event for an xdg_popup is only
 	ever sent once for the initial configuration. Starting with version 3,
 	it may be sent again if the popup is setup with an xdg_positioner with
-	set_reactive requested.
+	set_reactive requested, or in response to xdg_popup.reposition requests.
       </description>
       <arg name="x" type="int"
 	   summary="x position relative to parent surface window geometry"/>
@@ -1156,5 +1183,58 @@
       </description>
     </event>
 
+    <!-- Version 3 additions -->
+
+    <request name="reposition" since="3">
+      <description summary="recalculate the popup's location">
+	Reposition an already-mapped popup. The popup will be placed given the
+	details in the passed xdg_positioner object, and a
+	xdg_popup.repositioned followed by xdg_popup.configure and
+	xdg_surface.configure will be emitted in response. Any parameters set
+	by the previous positioner will be discarded.
+
+	The passed token will be sent in the corresponding
+	xdg_popup.repositioned event. The new popup position will not take
+	effect until the corresponding configure event is acknowledged by the
+	client. See xdg_popup.repositioned for details. The token itself is
+	opaque, and has no other special meaning.
+
+	If multiple reposition requests are sent, the compositor may skip all
+	but the last one.
+
+	If the popup is repositioned in response to a configure event for its
+	parent, the client should send an xdg_positioner.set_parent_configure
+	and possibly a xdg_positioner.set_parent_size request to allow the
+	compositor to properly constrain the popup.
+
+	If the popup is repositioned together with a parent that is being
+	resized, but not in response to a configure event, the client should
+	send a xdg_positioner.set_parent_size request.
+      </description>
+      <arg name="positioner" type="object" interface="xdg_positioner"/>
+      <arg name="token" type="uint" summary="reposition request token"/>
+    </request>
+
+    <event name="repositioned" since="3">
+      <description summary="signal the completion of a repositioned request">
+	The repositioned event is sent as part of a popup configuration
+	sequence, together with xdg_popup.configure and lastly
+	xdg_surface.configure to notify the completion of a reposition request.
+
+	The repositioned event is to notify about the completion of a
+	xdg_popup.reposition request. The token argument is the token passed
+	in the xdg_popup.reposition request.
+
+	Immediately after this event is emitted, xdg_popup.configure and
+	xdg_surface.configure will be sent with the updated size and position,
+	as well as a new configure serial.
+
+	The client should optionally update the content of the popup, but must
+	acknowledge the new popup configuration for the new position to take
+	effect. See xdg_surface.ack_configure for details.
+      </description>
+      <arg name="token" type="uint" summary="reposition request token"/>
+    </event>
+
   </interface>
 </protocol>