Refactor file path handling in JobThreadContext to support combined rename and ClassName changes

- Updated the logic to use the effective file path when handling script-to-script transitions, ensuring that the correct path is used even after a rename operation.
- Enhanced the test suite with a new test case for combined rename and ClassName changes, verifying that the extension change is applied correctly after a file rename.
- Improved comments for clarity on the renaming process and its implications for file handling during updates.

src/change_processor.rs

@@ -654,21 +654,29 @@ impl JobThreadContext {
                                         "ModuleScript" | "Script" | "LocalScript"
                                     );
 
-                                    if old_is_script && new_is_script && path.exists() {
+                                    // If a rename handler already moved the file,
+                                    // use the new path instead of the stale
+                                    // instigating_source path.
+                                    let effective_path = overridden_source_path
+                                        .as_deref()
+                                        .unwrap_or(path.as_path());
+
+                                    if old_is_script && new_is_script && effective_path.exists() {
                                         // Script-to-script transition: rename the file extension.
                                         // For directory-format scripts, the path is the directory
                                         // (e.g., src/MyModule/), not the init file inside. We must
                                         // find and rename the init file, not the directory.
                                         // Resolve the actual file to rename. For directories,
                                         // find the init file inside; for files, use directly.
-                                        let init_result = if path.is_dir() {
-                                            Self::find_init_file(path)
-                                                .map(|f| (f.clone(), path.to_path_buf()))
+                                        let init_result = if effective_path.is_dir() {
+                                            Self::find_init_file(effective_path)
+                                                .map(|f| (f.clone(), effective_path.to_path_buf()))
                                         } else {
                                             Some((
-                                                path.clone(),
-                                                path.parent()
-                                                    .unwrap_or(path.as_path())
+                                                effective_path.to_path_buf(),
+                                                effective_path
+                                                    .parent()
+                                                    .unwrap_or(effective_path)
                                                     .to_path_buf(),
                                             ))
                                         };

tests/tests/two_way_sync.rs

@@ -932,6 +932,64 @@ fn combined_classname_and_source_update() {
     });
 }
 
+/// Test 19b: Combined rename + ClassName change in a single update.
+/// The rename handler moves the file first (existing.luau → Renamed.luau),
+/// then the ClassName handler must use the renamed path to apply the extension
+/// change (Renamed.luau → Renamed.server.luau). Without the fix, the ClassName
+/// handler checks the OLD path which no longer exists, silently skipping the
+/// extension rename.
+#[test]
+fn combined_rename_and_classname_change() {
+    run_serve_test("syncback_write", |session, _redactions| {
+        let (session_id, _rs_id, existing_id) = get_rs_and_existing(&session);
+
+        let old_path = session.path().join("src").join("existing.luau");
+        // After rename + ClassName change: should end up as Renamed.server.luau
+        let final_path = session.path().join("src").join("Renamed.server.luau");
+        // Should NOT exist: renamed but without extension change
+        let renamed_only = session.path().join("src").join("Renamed.luau");
+
+        assert_file_exists(&old_path, "existing.luau before combined update");
+
+        let mut props = UstrMap::default();
+        props.insert(
+            ustr("Source"),
+            Some(Variant::String(
+                "-- After rename + class change".to_string(),
+            )),
+        );
+        send_update(
+            &session,
+            &session_id,
+            InstanceUpdate {
+                id: existing_id,
+                changed_name: Some("Renamed".to_string()),
+                changed_class_name: Some(ustr("Script")),
+                changed_properties: props,
+                changed_metadata: None,
+            },
+        );
+
+        assert_not_exists(&old_path, "Original file after combined rename+classname");
+        assert_not_exists(
+            &renamed_only,
+            "Renamed.luau should not exist — ClassName handler should have \
+             applied the .server extension",
+        );
+        assert_file_exists(
+            &final_path,
+            "Renamed.server.luau should exist after combined rename+classname",
+        );
+
+        let content = fs::read_to_string(&final_path).unwrap();
+        assert!(
+            content.contains("After rename + class change"),
+            "Source should be in the final file, got: {}",
+            content
+        );
+    });
+}
+
 // ---------------------------------------------------------------------------
 // Tests: Standalone → directory conversion & suffix-aware removal
 // ---------------------------------------------------------------------------