month-view: hide overflow popover on drag and drop

When the user starts a drag operation from the overflow popover,
it should hide itself, otherwise it can block dropping the event
below it. Currently, this is not what happens.

Fix that by making the overflow popover a drag and drop target,
and hide it when it detects drag movement.

https://bugzilla.gnome.org/show_bug.cgi?id=771663

src/gcal-month-view.c

@@ -838,9 +838,14 @@ gcal_month_view_drag_motion (GtkWidget      *widget,
 
   /* Setup the drag highlight */
   if (priv->dnd_cell != -1)
-    gtk_drag_highlight (widget);
+    {
+      gtk_drag_highlight (widget);
+      gtk_widget_hide (priv->overflow_popover);
+    }
   else
-    gtk_drag_unhighlight (widget);
+    {
+      gtk_drag_unhighlight (widget);
+    }
 
   /*
    * Sets the status of the drag - if it fails, sets the action to 0 and
@@ -950,6 +955,14 @@ gcal_month_view_drag_leave (GtkWidget      *widget,
   gtk_widget_queue_draw (widget);
 }
 
+static gboolean
+cancel_dnd_from_overflow_popover (GtkWidget *popover)
+{
+  gtk_widget_hide (popover);
+
+  return FALSE;
+}
+
 static void
 add_new_event_button_cb (GtkWidget *button,
                          gpointer   user_data)
@@ -1028,6 +1041,10 @@ gcal_month_view_init (GcalMonthView *self)
   priv->overflow_popover = gtk_popover_new (GTK_WIDGET (self));
   gtk_style_context_add_class (gtk_widget_get_style_context (priv->overflow_popover), "events");
   g_signal_connect (priv->overflow_popover, "hide", G_CALLBACK (overflow_popover_hide), self);
+  g_signal_connect_swapped (priv->overflow_popover,
+                            "drag-motion",
+                            G_CALLBACK (cancel_dnd_from_overflow_popover),
+                            priv->overflow_popover);
 
   grid = gtk_grid_new ();
   g_object_set (grid, "row-spacing", 6, "orientation", GTK_ORIENTATION_VERTICAL, NULL);
@@ -1053,6 +1070,17 @@ gcal_month_view_init (GcalMonthView *self)
                      NULL,
                      0,
                      GDK_ACTION_MOVE);
+
+  /*
+   * Also set the overflow popover as a drop destination, so we can hide
+   * it when the user starts dragging an event that is inside the overflow
+   * list.
+   */
+  gtk_drag_dest_set (GTK_WIDGET (priv->overflow_popover),
+                     0,
+                     NULL,
+                     0,
+                     GDK_ACTION_MOVE);
 }
 
 static void