[mutter/cherry-pick-70e44c28] backends/native: Ensure pointer is onscreen in input thread

[Jonas Ådahl <jadahl src gnome org>]

commit 5b87be8ce8dd09855b5367df92e44cc0e8aeb2ea
Author: Carlos Garnacho <carlosg gnome org>
Date:   Fri Jan 14 20:29:45 2022 +0100

    backends/native: Ensure pointer is onscreen in input thread
    
    It's slightly racy to have the main thread update the views and warp
    the pointer, since the input thread may not be aware yet of the new
    viewport layout.
    
    Make the input thread clamp the pointer so it remains onscreen
    instead, when the new viewports are obtained.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/64
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2237>
    
    
    (cherry picked from commit 70e44c281cb90d4f2dd23cd8b842bf0842e7407f)

 src/backends/native/meta-seat-impl.c | 55 ++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
---
diff --git a/src/backends/native/meta-seat-impl.c b/src/backends/native/meta-seat-impl.c
index 26db30bd42..28349789d3 100644
--- a/src/backends/native/meta-seat-impl.c
+++ b/src/backends/native/meta-seat-impl.c
@@ -3453,6 +3453,59 @@ meta_seat_impl_set_pointer_constraint (MetaSeatImpl              *seat_impl,
   g_object_unref (task);
 }
 
+static void
+ensure_pointer_onscreen (MetaSeatImpl *seat_impl)
+{
+  int i, candidate = -1;
+  int nearest_monitor_x, nearest_monitor_y, min_distance = G_MAXINT;
+  cairo_rectangle_int_t monitor_rect;
+  graphene_point_t coords;
+
+  if (!meta_seat_impl_query_state (seat_impl,
+                                   seat_impl->core_pointer, NULL,
+                                   &coords, NULL))
+    return;
+
+  /* Pointer is in a view */
+  if (meta_viewport_info_get_view_at (seat_impl->viewports,
+                                      coords.x, coords.y) >= 0)
+    return;
+
+  /* Find nearest view */
+  for (i = 0; i < meta_viewport_info_get_num_views (seat_impl->viewports); i++)
+    {
+      meta_viewport_info_get_view_info (seat_impl->viewports, i,
+                                        &monitor_rect, NULL);
+      nearest_monitor_x = MIN (ABS (coords.x - monitor_rect.x),
+                               ABS (coords.x -
+                                    monitor_rect.x + monitor_rect.width));
+      nearest_monitor_y = MIN (ABS (coords.y - monitor_rect.y),
+                               ABS (coords.y -
+                                    monitor_rect.y + monitor_rect.height));
+      if (nearest_monitor_x < min_distance ||
+          nearest_monitor_y < min_distance)
+        {
+          min_distance = MIN (nearest_monitor_x, nearest_monitor_y);
+          candidate = i;
+        }
+    }
+
+  if (candidate < 0)
+    return;
+
+  /* Calculate new coordinates on nearest view */
+  meta_viewport_info_get_view_info (seat_impl->viewports,
+                                    candidate,
+                                    &monitor_rect, NULL);
+  coords.x = CLAMP (coords.x, monitor_rect.x,
+                    monitor_rect.x + monitor_rect.width - 1);
+  coords.y = CLAMP (coords.y, monitor_rect.y,
+                    monitor_rect.y + monitor_rect.height - 1);
+
+  notify_absolute_motion_in_impl (seat_impl->core_pointer, 0,
+                                  coords.x, coords.y, NULL);
+}
+
 static gboolean
 set_viewports (GTask *task)
 {
@@ -3462,6 +3515,8 @@ set_viewports (GTask *task)
   g_set_object (&seat_impl->viewports, viewports);
   g_task_return_boolean (task, TRUE);
 
+  ensure_pointer_onscreen (seat_impl);
+
   return G_SOURCE_REMOVE;
 }