Fix UI node Transform change detection (#4138)

# Objective

Fixes #4133 

## Solution

Add comparisons to make sure we don't dereference `Mut<>` in the two places where `Transform` is being mutated. `GlobalTransform` implementation already works properly so fixing Transform automatically fixed that as well.

crates/bevy_ui/src/flex/mod.rs

@@ -278,17 +278,22 @@ pub fn flex_node_system(
             to_logical(layout.size.width),
             to_logical(layout.size.height),
         );
+        // only trigger change detection when the new value is different
         if node.size != new_size {
             node.size = new_size;
         }
-        let position = &mut transform.translation;
-        position.x = to_logical(layout.location.x + layout.size.width / 2.0);
-        position.y = to_logical(layout.location.y + layout.size.height / 2.0);
+        let mut new_position = transform.translation;
+        new_position.x = to_logical(layout.location.x + layout.size.width / 2.0);
+        new_position.y = to_logical(layout.location.y + layout.size.height / 2.0);
         if let Some(parent) = parent {
             if let Ok(parent_layout) = flex_surface.get_layout(parent.0) {
-                position.x -= to_logical(parent_layout.size.width / 2.0);
-                position.y -= to_logical(parent_layout.size.height / 2.0);
+                new_position.x -= to_logical(parent_layout.size.width / 2.0);
+                new_position.y -= to_logical(parent_layout.size.height / 2.0);
             }
         }
+        // only trigger change detection when the new value is different
+        if transform.translation != new_position {
+            transform.translation = new_position;
+        }
     }
 }

crates/bevy_ui/src/update.rs

@@ -45,7 +45,11 @@ fn update_hierarchy(
 ) -> f32 {
     current_global_z += UI_Z_STEP;
     if let Ok(mut transform) = node_query.get_mut(entity) {
-        transform.translation.z = current_global_z - parent_global_z;
+        let new_z = current_global_z - parent_global_z;
+        // only trigger change detection when the new value is different
+        if transform.translation.z != new_z {
+            transform.translation.z = new_z;
+        }
     }
     if let Ok(children) = children_query.get(entity) {
         let current_parent_global_z = current_global_z;