[mutter] clutter: Only pick on motion or touch update events

[Marge Bot <marge-bot src gnome org>]

commit 734a185915464d2da32b6b1251c8af9a631b0f8c
Author: Jonas Dreßler <verdre v0yd nl>
Date:   Thu Feb 11 17:34:07 2021 +0100

    clutter: Only pick on motion or touch update events
    
    Aside from ENTER/LEAVE, there are only two kinds of events that can move
    the pointer, motion events and touch update events. Everything else
    keeps the pointer at it's current position.
    
    The reason we pick inside _clutter_process_event_details() is that we
    want to set the event actor. Now if an event can't move the pointer, it
    also can't change the event actor (well, it can subsequently by
    triggering changes to the scenegraph, but that's handled elsewhere), so
    there's no need to pick a new event actor when we get those events.
    Instead, simply reuse the actor that's already associated with the
    current input device as the event actor for non MOTION/TOUCH_UPDATE
    events.
    
    Events where a device or a touchpoint goes away (like DEVICE_REMOVED or
    TOUCH_END/CANCEL) also affect picking, they don't need a repick, but
    instead the actor associated with the device/touchpoint needs to be
    unassociated. This is ensured by invoking remove_device_for_event() on
    those events and will not be affected by this change.
    
    This should improve performance while scrolling quite a bit, since
    scroll events come in unthrottled and we now no longer do a repick on
    each one of those.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1729>

 clutter/clutter/clutter-main.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)
---
diff --git a/clutter/clutter/clutter-main.c b/clutter/clutter/clutter-main.c
index 36bcd88e5c..1d62d590c6 100644
--- a/clutter/clutter/clutter-main.c
+++ b/clutter/clutter/clutter-main.c
@@ -1701,8 +1701,18 @@ _clutter_process_event_details (ClutterActor        *stage,
                   break;
                 }
 
-              event->any.source =
-                update_device_for_event (CLUTTER_STAGE (stage), event, TRUE);
+              if (event->type == CLUTTER_MOTION)
+                {
+                  event->any.source =
+                    update_device_for_event (CLUTTER_STAGE (stage), event, TRUE);
+                }
+              else
+                {
+                  event->any.source =
+                    clutter_stage_get_device_actor (CLUTTER_STAGE (stage),
+                                                    device,
+                                                    NULL);
+                }
 
               if (event->any.source == NULL)
                 break;
@@ -1796,8 +1806,18 @@ _clutter_process_event_details (ClutterActor        *stage,
                   break;
                 }
 
-              event->any.source =
-                update_device_for_event (CLUTTER_STAGE (stage), event, TRUE);
+              if (event->type == CLUTTER_TOUCH_UPDATE)
+                {
+                  event->any.source =
+                    update_device_for_event (CLUTTER_STAGE (stage), event, TRUE);
+                }
+              else
+                {
+                  event->any.source =
+                    clutter_stage_get_device_actor (CLUTTER_STAGE (stage),
+                                                    device,
+                                                    event->touch.sequence);
+                }
 
               if (event->any.source == NULL)
                 break;