UI: Fix Editor Highlight Top Bar

New editor highlighting (#116684) will sometimes highlight a nearby
area when hovering over the Top Bar. This is caused by the use of the
top-left corner for testing. This just uses the region midpoint
instead.

Pull Request: https://projects.blender.org/blender/blender/pulls/128394

source/blender/editors/screen/screen_draw.cc

@@ -174,7 +174,7 @@ void ED_screen_draw_edges(wmWindow *win)
 
   if (region) {
     /* Find active area from active region. */
-    const int pos[2] = {region->winrct.xmin, region->winrct.ymin};
+    const int pos[2] = {BLI_rcti_cent_x(&region->winrct), BLI_rcti_cent_y(&region->winrct)};
     active_area = BKE_screen_find_area_xy(screen, SPACE_TYPE_ANY, pos);
   }