dnd: Add addClickAction() method

The events that the draggable uses to initiate a drag operation
are the same as the ones a click action consumes for clicks.

It is still possible to combine the two, but it's finicky and far
from being straight-forward. To make this easier, add a dedicated
addClickAction() method to draggables that takes care of the
setup before adding the action to the draggable actor.

In the longer term, we'll want to turn DND into an action, and
have something like GTK's gesture group to allow combining actions
that would otherwise step on each other's toes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2742>

js/ui/dnd.js

@@ -119,6 +119,50 @@ var _Draggable = class _Draggable extends Signals.EventEmitter {
         this._dragCancellable = true;
     }
 
+    /**
+     * addClickAction:
+     *
+     * @param {Clutter.ClickAction} action - click action to add to draggable actor
+     *
+     * Add @action to the draggable's actor, and set it up so that it does not
+     * impede drag operations.
+     */
+    addClickAction(action) {
+        action.connect('clicked', () => (this._actionClicked = true));
+        action.connect('long-press', (a, actor, state) => {
+            if (state !== Clutter.LongPressState.CANCEL)
+                return true;
+
+            const event = Clutter.get_current_event();
+            this._dragTouchSequence = event.get_event_sequence();
+
+            if (this._longPressLater)
+                return true;
+
+            // A click cancels a long-press before any click handler is
+            // run - make sure to not start a drag in that case
+            const laters = global.compositor.get_laters();
+            this._longPressLater = laters.add(Meta.LaterType.BEFORE_REDRAW, () => {
+                delete this._longPressLater;
+                if (this._actionClicked) {
+                    delete this._actionClicked;
+                    return GLib.SOURCE_REMOVE;
+                }
+                action.release();
+                this.startDrag(
+                    ...action.get_coords(),
+                    event.get_time(),
+                    this._dragTouchSequence,
+                    event.get_device());
+
+                return GLib.SOURCE_REMOVE;
+            });
+            return true;
+        });
+
+        this.actor.add_action(action);
+    }
+
     _onButtonPress(actor, event) {
         if (event.get_button() != 1)
             return Clutter.EVENT_PROPAGATE;