Fix syncback not removing stale $properties at engine defaults (#1244)

CHANGELOG.md

@@ -45,6 +45,7 @@ Making a new release? Simply add the new header with the version and date undern
 * Instances that share a name and class are now robustly matched on resync by comparing their properties, instead of relying on child order alone. ([#1266])
 * Rojo now reports a clear error instead of panicking in several cases, including when the `serve` port is already in use, when a synced file is read-only or locked, when the filesystem watcher can't be created, and when the working directory is inaccessible. ([#1267])
 * `rojo serve` now validates the `Host`/`Origin` headers to protect the local/private server against DNS rebinding, gates `/api/open` to local clients, and warns when bound to a network-reachable address. The accepted hosts can be extended with the `--allowed-hosts` option or a project's `serveAllowedHosts` field, for example to reach a network-exposed server by hostname. ([#1270])
+* Fixed syncback not removing stale `$properties` entries when Studio resets a property to its engine default. ([#1244])
 
 [#1176]: https://github.com/rojo-rbx/rojo/pull/1176
 [#1179]: https://github.com/rojo-rbx/rojo/pull/1179
@@ -60,6 +61,7 @@ Making a new release? Simply add the new header with the version and date undern
 [#1266]: https://github.com/rojo-rbx/rojo/pull/1266
 [#1267]: https://github.com/rojo-rbx/rojo/pull/1267
 [#1270]: https://github.com/rojo-rbx/rojo/pull/1270
+[#1244]: https://github.com/rojo-rbx/rojo/pull/1244
 
 ## [7.7.0-rc.1] (November 27th, 2025)
 

rojo-test/syncback-test-snapshots/end_to_end__rojo_test__syncback_util__project_default_properties_remove-stdout.snap

@@ -0,0 +1,5 @@
+---
+source: tests/rojo_test/syncback_util.rs
+expression: "String::from_utf8_lossy(&output.stdout)"
+---
+Writing default.project.json

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__project_default_properties_remove-default.project.json.snap

@@ -0,0 +1,59 @@
+---
+source: tests/tests/syncback.rs
+expression: default.project.json
+---
+{
+  "name": "SyncbackTest",
+  "tree": {
+    "$className": "DataModel",
+    "Workspace": {
+      "$className": "Workspace",
+      "TestPart": {
+        "$className": "Part",
+        "$properties": {
+          "Anchored": true,
+          "Color": {
+            "Color3uint8": [
+              0,
+              0,
+              255
+            ]
+          }
+        }
+      },
+      "$properties": {
+        "EnableSLIMAvatars": {
+          "Enum": 0
+        },
+        "ImprovedAnimationConstraint": {
+          "Enum": 0
+        },
+        "ImprovedPhysicsReplication": {
+          "Enum": 0
+        },
+        "LayeredClothingCacheOptimizations": {
+          "Enum": 0
+        },
+        "MeshStreamingAndImprovedLods": {
+          "Enum": 0
+        },
+        "NextGenerationReplication": {
+          "Enum": 0
+        },
+        "PlayerScriptsUseInputActionSystem": {
+          "Enum": 0
+        },
+        "UseFixedSimulation": {
+          "Enum": 0
+        },
+        "UseNewLuauTypeSolver": "Disabled",
+        "ValidateEnabledProximityPrompt": {
+          "Enum": 0
+        }
+      },
+      "$attributes": {
+        "Rojo_Target_CurrentCamera": "302d573157260ee80a3baa32000003b5"
+      }
+    }
+  }
+}

rojo-test/syncback-tests/project_default_properties_remove/input-project/default.project.json

@@ -0,0 +1,21 @@
+{
+  "name": "SyncbackTest",
+  "tree": {
+    "$className": "DataModel",
+    "Workspace": {
+      "$className": "Workspace",
+      "TestPart": {
+        "$className": "Part",
+        "$properties": {
+          "Transparency": 1.0,
+          "Anchored": true,
+          "Color": [
+            1.0,
+            0.0,
+            0.0
+          ]
+        }
+      }
+    }
+  }
+}

src/snapshot_middleware/project.rs

@@ -344,6 +344,11 @@ pub fn syncback_project<'sync>(
     let mut new_child_map = HashMap::new();
 
     let mut node_changed_map = Vec::new();
+    // Tracks whether any stale default-valued properties were removed from
+    // project nodes. If so, we must reserialize even if
+    // project_node_should_reserialize wouldn't otherwise detect a change
+    // (it only compares node properties forward, not in reverse).
+    let mut removed_stale_properties = false;
     let mut node_queue = VecDeque::with_capacity(1);
     node_queue.push_back((&mut project.tree, old_inst, snapshot.new_inst()));
 
@@ -402,9 +407,11 @@ pub fn syncback_project<'sync>(
 
             // We only want to set properties if it needs it.
             if !middleware.handles_own_properties() {
+                removed_stale_properties |=
                     project_node_property_syncback_path(snapshot, new_inst, node);
             }
         } else {
+            removed_stale_properties |=
                 project_node_property_syncback_no_path(snapshot, new_inst, node);
         }
 
@@ -507,12 +514,18 @@ pub fn syncback_project<'sync>(
     }
     let mut fs_snapshot = FsSnapshot::new();
 
+    let mut needs_reserialize = removed_stale_properties;
+    if !needs_reserialize {
         for (node_properties, node_attributes, old_inst) in node_changed_map {
             if project_node_should_reserialize(node_properties, node_attributes, old_inst)? {
-            fs_snapshot.add_file(project_path, serde_json::to_vec_pretty(&project)?);
+                needs_reserialize = true;
                 break;
             }
         }
+    }
+    if needs_reserialize {
+        fs_snapshot.add_file(project_path, serde_json::to_vec_pretty(&project)?);
+    }
 
     Ok(SyncbackReturn {
         fs_snapshot,
@@ -521,15 +534,18 @@ pub fn syncback_project<'sync>(
     })
 }
 
+/// Syncs properties from the new instance into the project node.
+/// Returns `true` if any stale properties were removed (i.e. properties
+/// that existed in the project node but are now at their engine default).
 fn project_node_property_syncback(
     _snapshot: &SyncbackSnapshot,
     filtered_properties: UstrMap<&Variant>,
     new_inst: &Instance,
     node: &mut ProjectNode,
-) {
+) -> bool {
     let properties = &mut node.properties;
     let mut attributes = BTreeMap::new();
-    for (name, value) in filtered_properties {
+    for (&name, &value) in &filtered_properties {
         match value {
             Variant::Attributes(attrs) => {
                 for (attr_name, attr_value) in attrs.iter() {
@@ -552,14 +568,48 @@ fn project_node_property_syncback(
             }
         }
     }
+
+    // Remove stale properties: entries that exist in the project node's
+    // $properties but are no longer in the filtered (non-default) properties
+    // from the instance. This handles the case where Studio resets a property
+    // to its engine default — filter_properties won't include it, so we need
+    // to clean up the now-stale project entry.
+    let class_data = rbx_reflection_database::get()
+        .ok()
+        .and_then(|db| db.classes.get(new_inst.class.as_str()));
+    let len_before = properties.len();
+    properties.retain(|prop_name, _| {
+        if filtered_properties.contains_key(prop_name) {
+            return true;
+        }
+        // Only remove if the property has a known default value in the
+        // reflection database. If there's no default, the property might be
+        // absent from the instance for other reasons (e.g. unknown property),
+        // so we conservatively keep it.
+        if let Some(data) = &class_data {
+            if data.default_properties.contains_key(prop_name.as_str()) {
+                log::debug!(
+                    "Removing stale property '{}' from project node for class '{}': \
+                     value has been reset to engine default",
+                    prop_name,
+                    new_inst.class
+                );
+                return false;
+            }
+        }
+        true
+    });
+    let removed_stale = properties.len() < len_before;
+
     node.attributes = attributes;
+    removed_stale
 }
 
 fn project_node_property_syncback_path(
     snapshot: &SyncbackSnapshot,
     new_inst: &Instance,
     node: &mut ProjectNode,
-) {
+) -> bool {
     let filtered_properties = snapshot
         .get_path_filtered_properties(new_inst.referent())
         .unwrap();
@@ -570,7 +620,7 @@ fn project_node_property_syncback_no_path(
     snapshot: &SyncbackSnapshot,
     new_inst: &Instance,
     node: &mut ProjectNode,
-) {
+) -> bool {
     let filtered_properties = filter_properties(snapshot.project(), new_inst);
     project_node_property_syncback(snapshot, filtered_properties, new_inst, node)
 }

tests/tests/syncback.rs

@@ -86,4 +86,7 @@ syncback_tests! {
     sync_rules => ["src/module.modulescript", "src/text.text"],
     // Ensures that the `syncUnscriptable` setting works
     unscriptable_properties => ["default.project.json"],
+    // Ensures that syncback correctly removes default values from projects rather
+    // than leaving an incorrect value.
+    project_default_properties_remove => ["default.project.json"],
 }