widget: Propagate touch/button press events when a sequence is denied

If the captured touch begin or button press event have been consumed
for the given sequence, propagate it upwards if the sequence goes from
claimed to denied, so the widgets on the way to the event widget receive
a coherent event stream now that they're going to receive events.

gtk/gtkwidget.c

@@ -66,6 +66,7 @@
 #include "gtktypebuiltins.h"
 #include "a11y/gtkwidgetaccessible.h"
 #include "gtkapplicationprivate.h"
+#include "gtkgestureprivate.h"
 
 /* for the use of round() */
 #include "fallback-c89.c"
@@ -4044,14 +4045,90 @@ gtk_widget_get_property (GObject         *object,
     }
 }
 
+static void
+_gtk_widget_emulate_press (GtkWidget      *widget,
+                           const GdkEvent *event)
+{
+  GtkWidget *event_widget, *next_child, *parent;
+  GdkEvent *press;
+
+  event_widget = gtk_get_event_widget ((GdkEvent *) event);
+
+  if (event_widget == widget)
+    return;
+
+  if (event->type == GDK_TOUCH_BEGIN ||
+      event->type == GDK_TOUCH_UPDATE ||
+      event->type == GDK_TOUCH_END)
+    {
+      press = gdk_event_copy (event);
+      press->type = GDK_TOUCH_BEGIN;
+    }
+  else if (event->type == GDK_BUTTON_PRESS ||
+           event->type == GDK_BUTTON_RELEASE)
+    {
+      press = gdk_event_copy (event);
+      press->type = GDK_BUTTON_PRESS;
+    }
+  else if (event->type == GDK_MOTION_NOTIFY)
+    {
+      press = gdk_event_new (GDK_BUTTON_PRESS);
+      press->button.window = g_object_ref (event->motion.window);
+      press->button.time = event->motion.time;
+      press->button.x = event->motion.x;
+      press->button.y = event->motion.y;
+      press->button.x_root = event->motion.x_root;
+      press->button.y_root = event->motion.y_root;
+      press->button.state = event->motion.state;
+
+      press->button.axes = g_memdup (event->motion.axes,
+                                     sizeof (gdouble) *
+                                     gdk_device_get_n_axes (event->motion.device));
+
+      if (event->motion.state & GDK_BUTTON3_MASK)
+        press->button.button = 3;
+      else if (event->motion.state & GDK_BUTTON2_MASK)
+        press->button.button = 2;
+      else
+        {
+          if ((event->motion.state & GDK_BUTTON1_MASK) == 0)
+            g_critical ("Guessing button number 1 on generated button press event");
+
+          press->button.button = 1;
+        }
+
+      gdk_event_set_device (press, gdk_event_get_device (event));
+      gdk_event_set_source_device (press, gdk_event_get_source_device (event));
+    }
+  else
+    return;
+
+  press->any.send_event = TRUE;
+  next_child = event_widget;
+  parent = gtk_widget_get_parent (next_child);
+
+  while (parent != widget)
+    {
+      next_child = parent;
+      parent = gtk_widget_get_parent (parent);
+    }
+
+  /* Perform propagation state starting from the next child in the chain */
+  if (!_gtk_propagate_captured_event (event_widget, press, next_child))
+    gtk_propagate_event (event_widget, press);
+
+  gdk_event_free (press);
+}
+
 static gboolean
 _gtk_widget_set_sequence_state_internal (GtkWidget             *widget,
                                          GdkEventSequence      *sequence,
                                          GtkEventSequenceState  state)
 {
   GtkWidgetPrivate *priv = widget->priv;
+  const GdkEvent *mimic_event = NULL;
+  gboolean retval, handled = FALSE;
   EventControllerData *data;
-  gboolean handled = FALSE;
   GList *l;
 
   for (l = priv->event_controllers; l; l = l->next)
@@ -4061,10 +4138,27 @@ _gtk_widget_set_sequence_state_internal (GtkWidget             *widget,
       if (!GTK_IS_GESTURE (data->controller))
         continue;
 
-      handled |= gtk_gesture_set_sequence_state (GTK_GESTURE (data->controller),
-                                                 sequence, state);
+      retval = gtk_gesture_set_sequence_state (GTK_GESTURE (data->controller),
+                                               sequence, state);
+      handled |= retval;
+
+      /* If the sequence goes denied, check whether this is a controller attached
+       * to the capture phase, that additionally handled the button/touch press (ie.
+       * it was consumed), the corresponding press will be emulated for widgets
+       * beneath, so the widgets beneath get a coherent stream of events from now on.
+       */
+      if (retval && !mimic_event &&
+          data->propagation_phase == GTK_PHASE_CAPTURE &&
+          state == GTK_EVENT_SEQUENCE_DENIED &&
+          _gtk_gesture_handled_sequence_press (GTK_GESTURE (data->controller),
+                                               sequence))
+        mimic_event = gtk_gesture_get_last_event (GTK_GESTURE (data->controller),
+                                                  sequence);
     }
 
+  if (mimic_event)
+    _gtk_widget_emulate_press (widget, mimic_event);
+
   return handled;
 }