fix: the cross-kind refusal reads properly, and a session flow keeps its own response's head

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/drafts_move_taken.rs

@@ -215,7 +215,23 @@ async fn test_draft_move_refuses_another_users_other_app_kind(
         "a classic app was moved onto another user's raw app: {body}"
     );
     assert!(
-        body.contains("Another user has a raw app draft"),
+        body.contains("holds another user's raw app draft"),
+        "the refusal did not name the occupant: {body}"
+    );
+
+    // And the other direction, where the occupant reads as the classic kind.
+    let (status, body) = move_to(
+        "raw_app",
+        "u/test-user/mvtaken_raw",
+        "u/test-user/mvtaken_app_theirs",
+    )
+    .await?;
+    assert_eq!(
+        status, 400,
+        "a raw app was moved onto another user's classic app: {body}"
+    );
+    assert!(
+        body.contains("holds another user's app draft"),
         "the refusal did not name the occupant: {body}"
     );
 

backend/tests/fixtures/drafts_move_taken.sql

@@ -32,4 +32,6 @@ INSERT INTO draft (workspace_id, path, typ, value, email) VALUES
     ('test-workspace', 'u/test-user/mvtaken_raw', 'raw_app',
      '{"summary": "raw", "files": {}}', 'test@windmill.dev'),
     ('test-workspace', 'u/test-user/mvtaken_theirs', 'raw_app',
-     '{"summary": "theirs", "files": {}}', 'test2@windmill.dev');
+     '{"summary": "theirs", "files": {}}', 'test2@windmill.dev'),
+    ('test-workspace', 'u/test-user/mvtaken_app_theirs', 'app',
+     '{"summary": "theirs", "value": {}}', 'test2@windmill.dev');

backend/windmill-api/src/drafts.rs

@@ -859,8 +859,8 @@ async fn move_draft(
                 // The other app kind, owned by someone else: one deployed path cannot hold
                 // both, so this is the other item's path, not a teammate's copy of this one.
                 format!(
-                    "Another user has a {occupant} draft at '{new_path}', and an app and a \
-                     raw app cannot share a path."
+                    "'{new_path}' holds another user's {occupant} draft, and an app and a raw \
+                     app cannot share a path."
                 )
             }
         } else {

frontend/src/lib/components/sessions/sessionRuntime.svelte.ts

@@ -593,8 +593,9 @@ function createRuntime(session: Session): SessionRuntime {
 				// when the path has never been deployed.
 				const aiDraft = UserDraft.get<Flow>('flow', path, { workspace })
 
-				// getDraft=true omits version_id (the plain getFlowByPath has it) —
-				// stamp it on so the flow doesn't always diff. Best-effort.
+				// Fallback for the head: the payload fetches below carry `version_id`, and
+				// this one covers a response that does not. Best-effort, and a request of
+				// its own, so the same response wins wherever both are available.
 				let deployedVersionId: number | undefined
 				try {
 					deployedVersionId = (await FlowService.getFlowByPath({ workspace, path }))?.version_id
@@ -608,9 +609,12 @@ function createRuntime(session: Session): SessionRuntime {
 					// yet on the backend — draft-only flows are a valid state.
 					try {
 						const result = await FlowService.getFlowByPath({ workspace, path, getDraft: true })
-						// `getDraft` omits `version_id`; the editor's deploy guard compares
-						// against it, so put the head fetched above back on the baseline.
-						saved.val = { ...(result as SavedFlow), version_id: deployedVersionId }
+						// The editor's deploy guard compares against the head, so keep this
+						// response's own and fall back to the one fetched above.
+						saved.val = {
+							...(result as SavedFlow),
+							version_id: (result as SavedFlow).version_id ?? deployedVersionId
+						}
 					} catch {
 						saved.val = undefined
 					}
@@ -622,7 +626,10 @@ function createRuntime(session: Session): SessionRuntime {
 
 				// No local draft yet — seed from `result.draft ?? result`.
 				const result = await FlowService.getFlowByPath({ workspace, path, getDraft: true })
-				saved.val = { ...(result as SavedFlow), version_id: deployedVersionId }
+				saved.val = {
+					...(result as SavedFlow),
+					version_id: (result as SavedFlow).version_id ?? deployedVersionId
+				}
 				const serverDraft = (result as SavedFlow).draft as Flow | undefined
 				const flow: Flow = (serverDraft ?? (result as Flow)) as Flow
 				// Seed the per-tab last_sync from the server draft's timestamp so the