WM: prevent drag events being continually tested

Click-drag events that weren't handled would continually be tested
for each mouse-motion event.

As well as being redundant, this added the overhead of querying
gizmos twice per motion event.

Now click-drag is only tested once when the drag threshold is reached.

This mitigates T87511, although the single drag test still causes
the snap gizmo to flicker.

2 files changed, 11 insertions, 0 deletions

diff --git a/source/blender/windowmanager/intern/wm.c b/source/blender/windowmanager/intern/wm.c
index ae5b6c468f7..46e47ae95c4 100644
--- a/source/blender/windowmanager/intern/wm.c
+++ b/source/blender/windowmanager/intern/wm.c
@@ -186,6 +186,7 @@ static void window_manager_blend_read_data(BlendDataReader *reader, ID *id)
     win->addmousemove = true;
     win->event_queue_check_click = 0;
     win->event_queue_check_drag = 0;
+    win->event_queue_check_drag_handled = 0;
     BLO_read_data_address(reader, &win->stereo3d_format);
 
     /* Multi-view always fallback to anaglyph at file opening
diff --git a/source/blender/windowmanager/intern/wm_event_system.c b/source/blender/windowmanager/intern/wm_event_system.c
index 627ec1d5722..cb4aa3b6549 100644
--- a/source/blender/windowmanager/intern/wm_event_system.c
+++ b/source/blender/windowmanager/intern/wm_event_system.c
@@ -2941,6 +2941,8 @@ static int wm_handlers_do(bContext *C, wmEvent *event, ListBase *handlers)
     if (wm_action_not_handled(action)) {
       if (win->event_queue_check_drag) {
         if (WM_event_drag_test(event, &event->prevclickx)) {
+          win->event_queue_check_drag_handled = true;
+
           int x = event->x;
           int y = event->y;
           short val = event->val;
@@ -2984,6 +2986,7 @@ static int wm_handlers_do(bContext *C, wmEvent *event, ListBase *handlers)
         if (event->is_repeat == false) {
           win->event_queue_check_click = true;
           win->event_queue_check_drag = true;
+          win->event_queue_check_drag_handled = false;
         }
       }
       else if (event->val == KM_RELEASE) {
@@ -3470,6 +3473,13 @@ void wm_event_do_handlers(bContext *C)
         win->event_queue_check_click = false;
       }
 
+      /* If the drag even was handled, don't attempt to keep re-handing the same
+       * drag event on every cursor motion, see: T87511. */
+      if (win->event_queue_check_drag_handled) {
+        win->event_queue_check_drag = false;
+        win->event_queue_check_drag_handled = false;
+      }
+
       /* Update previous mouse position for following events to use. */
       win->eventstate->prevx = event->x;
       win->eventstate->prevy = event->y;