fix: a saved layout that stacks nodes is not a usable layout

packages/diagram-server/src/server/source-model-storage.ts

@@ -98,8 +98,38 @@ const RUN_OUTPUT_FALLBACK_MAX_RESULTS = 50;
  * a diagram they can read, the result is persisted immediately so it happens
  * once, and this failure is at least visible where the other was silent.
  */
-export function savedLayoutIsUsable(hasSavedPositions: boolean, unpositionedNodeCount: number): boolean {
-    return hasSavedPositions && unpositionedNodeCount === 0;
+export function savedLayoutIsUsable(
+    hasSavedPositions: boolean,
+    unpositionedNodeCount: number,
+    coincidentNodeCount = 0
+): boolean {
+    return hasSavedPositions && unpositionedNodeCount === 0 && coincidentNodeCount === 0;
+}
+
+/**
+ * How many nodes share a position with another node.
+ *
+ * Covering every node turned out not to be enough. A layout captured before its
+ * nodes were ever placed names all of them and puts several at the same point —
+ * the origin, usually, since that is where a node starts. It passes a coverage
+ * check, suppresses the layout run, and is restored faithfully on every open,
+ * so the diagram opens stacked forever and only a layout by hand fixes it.
+ *
+ * Two nodes at the same coordinates is the reliable signature. No layout engine
+ * ever produces it, and nobody drags one node exactly on top of another, so it
+ * means the positions were recorded rather than computed.
+ */
+export function countCoincidentNodes(positions: Iterable<{ x: number; y: number }>): number {
+    const seen = new Set<string>();
+    let coincident = 0;
+    for (const { x, y } of positions) {
+        const key = `${x},${y}`;
+        if (seen.has(key)) {
+            coincident++;
+        }
+        seen.add(key);
+    }
+    return coincident;
 }
 
 /**
@@ -484,7 +514,8 @@ export class WorkflowSourceModelStorage implements SourceModelStorage {
 
         const hasPersistedLayout = savedLayoutIsUsable(
             loadedLayoutPositions !== undefined,
-            unpositionedNodeCount
+            unpositionedNodeCount,
+            countCoincidentNodes(loadedLayoutPositions?.values() ?? [])
         );
         const hasPersistedEdgeRoutesEffective = hasPersistedLayout && hasPersistedEdgeRoutes;
 

packages/diagram-server/test/saved-layout-coverage.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest';
-import { savedLayoutIsUsable } from '../src/server/source-model-storage';
+import { countCoincidentNodes, savedLayoutIsUsable } from '../src/server/source-model-storage';
 
 /**
  * When a saved layout may stand, and when the diagram must be laid out again.
@@ -42,4 +42,52 @@ describe('savedLayoutIsUsable', () => {
         // by a different runtime names nothing in this graph.
         expect(savedLayoutIsUsable(true, 9)).toBe(false);
     });
+
+    it('is false when the layout stacks nodes on one another', () => {
+        // Covering every node is not enough. A layout captured before its nodes
+        // were placed names all of them and puts several at the same point, so
+        // it passes coverage, suppresses the run, and is restored faithfully
+        // every time — the diagram opens stacked forever, and only a layout by
+        // hand ever fixes it.
+        expect(savedLayoutIsUsable(true, 0, 1)).toBe(false);
+        expect(savedLayoutIsUsable(true, 0, 0)).toBe(true);
+    });
+});
+
+describe('countCoincidentNodes', () => {
+    it('counts a node sharing a position with another', () => {
+        expect(
+            countCoincidentNodes([
+                { x: 0, y: 0 },
+                { x: 0, y: 0 },
+                { x: 10, y: 4 }
+            ])
+        ).toBe(1);
+    });
+
+    it('is zero for a layout that placed everything apart', () => {
+        expect(
+            countCoincidentNodes([
+                { x: 0, y: 0 },
+                { x: 10, y: 0 },
+                { x: 0, y: 10 }
+            ])
+        ).toBe(0);
+    });
+
+    it('is zero for nothing at all', () => {
+        // No saved positions is a different case, already handled by the
+        // coverage check, and must not be mistaken for a stacked layout.
+        expect(countCoincidentNodes([])).toBe(0);
+    });
+
+    it('counts every extra at a shared point, not just the pair', () => {
+        expect(
+            countCoincidentNodes([
+                { x: 5, y: 5 },
+                { x: 5, y: 5 },
+                { x: 5, y: 5 }
+            ])
+        ).toBe(2);
+    });
 });