display: Add meta_display_is_pointer_emulating_sequence()

This function tells the obvious on X11, and implements a similar mechanism
on wayland to determine the "pointer emulating" sequence, or one to stick
with when implementing single-touch behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=733631

src/core/display-private.h

@@ -253,6 +253,7 @@ struct _MetaDisplay
   MetaCompositor *compositor;
 
   MetaGestureTracker *gesture_tracker;
+  ClutterEventSequence *pointer_emulating_sequence;
 
   int composite_event_base;
   int composite_error_base;

src/core/display.c

@@ -3239,3 +3239,23 @@ meta_display_request_restart (MetaDisplay *display)
 
   return result;
 }
+
+/**
+ * meta_display_is_pointer_emulating_sequence:
+ * @display: the display
+ * @sequence: (nullable): a #ClutterEventSequence
+ *
+ * Tells whether the event sequence is the used for pointer emulation
+ * and single-touch interaction.
+ *
+ * Returns: #TRUE if the sequence emulates pointer behavior
+ **/
+gboolean
+meta_display_is_pointer_emulating_sequence (MetaDisplay          *display,
+                                            ClutterEventSequence *sequence)
+{
+  if (!sequence)
+    return FALSE;
+
+  return display->pointer_emulating_sequence == sequence;
+}

src/core/events.c

@@ -99,6 +99,62 @@ handle_idletime_for_event (const ClutterEvent *event)
 #endif /* HAVE_NATIVE_BACKEND */
 }
 
+static gboolean
+sequence_is_pointer_emulated (MetaDisplay        *display,
+                              const ClutterEvent *event)
+{
+  ClutterEventSequence *sequence;
+  MetaBackend *backend;
+
+  sequence = clutter_event_get_event_sequence (event);
+
+  if (!sequence)
+    return FALSE;
+
+  if (clutter_event_is_pointer_emulated (event))
+    return TRUE;
+
+  backend = meta_get_backend ();
+
+  /* When using Clutter's native input backend there is no concept of
+   * pointer emulating sequence, we still must make up our own to be
+   * able to implement single-touch (hence pointer alike) behavior.
+   *
+   * This is implemented similarly to X11, where only the first touch
+   * on screen gets the "pointer emulated" flag, and it won't get assigned
+   * to another sequence until the next first touch on an idle touchscreen.
+   */
+  if (META_IS_BACKEND_NATIVE (backend))
+    {
+      MetaGestureTracker *tracker;
+
+      tracker = meta_display_get_gesture_tracker (display);
+
+      if (event->type == CLUTTER_TOUCH_BEGIN &&
+          meta_gesture_tracker_get_n_current_touches (tracker) == 0)
+        return TRUE;
+    }
+
+  return FALSE;
+}
+
+static void
+meta_display_update_pointer_emulating_sequence (MetaDisplay        *display,
+                                                const ClutterEvent *event)
+{
+  ClutterEventSequence *sequence;
+
+  sequence = clutter_event_get_event_sequence (event);
+
+  if (event->type == CLUTTER_TOUCH_BEGIN &&
+      !display->pointer_emulating_sequence &&
+      sequence_is_pointer_emulated (display, event))
+    display->pointer_emulating_sequence = sequence;
+  else if (event->type == CLUTTER_TOUCH_END &&
+           display->pointer_emulating_sequence == sequence)
+    display->pointer_emulating_sequence = NULL;
+}
+
 static gboolean
 meta_display_handle_event (MetaDisplay        *display,
                            const ClutterEvent *event)
@@ -108,6 +164,8 @@ meta_display_handle_event (MetaDisplay        *display,
   MetaWaylandCompositor *compositor = NULL;
   MetaGestureTracker *tracker;
 
+  meta_display_update_pointer_emulating_sequence (display, event);
+
   if (meta_is_wayland_compositor ())
     {
       compositor = meta_wayland_compositor_get_default ();

src/meta/display.h

@@ -182,4 +182,7 @@ void meta_display_ungrab_keyboard (MetaDisplay *display,
                                    guint32      timestamp);
 void meta_display_unfreeze_keyboard (MetaDisplay *display,
                                      guint32      timestamp);
+gboolean meta_display_is_pointer_emulating_sequence (MetaDisplay          *display,
+                                                     ClutterEventSequence *sequence);
+
 #endif