test(bases): pin resolveCardDrop catch-fallback behavior + fix comment

apps/client/src/features/base/hooks/__tests__/resolve-card-drop.spec.ts

@@ -1,8 +1,14 @@
 import { describe, it, expect, vi } from "vitest";
 
+let throwOnNextCall = false;
 vi.mock("fractional-indexing-jittered", () => ({
-  generateJitteredKeyBetween: (a: string | null, b: string | null) =>
+  generateJitteredKeyBetween: (a: string | null, b: string | null) => {
-    `${a ?? "START"}|${b ?? "END"}`,
+    if (throwOnNextCall) {
+      throwOnNextCall = false;
+      throw new Error("lower >= upper");
+    }
+    return `${a ?? "START"}|${b ?? "END"}`;
+  },
 }));
 
 import { resolveCardDrop } from "../resolve-card-drop";
@@ -105,4 +111,22 @@ describe("resolveCardDrop", () => {
     expect(result.cells).toBeUndefined();
     expect(result.position).toBeUndefined();
   });
+
+  it("falls back to append-after-lower when generateJitteredKeyBetween throws", () => {
+    throwOnNextCall = true;
+    const result = resolveCardDrop({
+      draggedCardId: "r1",
+      targetCardId: "r2",
+      edge: "top",
+      sourceColumnKey: "c1",
+      targetColumnKey: "c1",
+      groupByPropertyId: "prop-status",
+      columnRows: [mkRow("r2", "b"), mkRow("r3", "d")],
+      sortsActive: false,
+    });
+    // First call (lower=null, upper="b") threw → second call (lower=null,
+    // upper=null) succeeds. The mock returns "START|END" in that case.
+    expect(result.cells).toBeUndefined();
+    expect(result.position).toBe("START|END");
+  });
 });

apps/client/src/features/base/hooks/resolve-card-drop.ts

@@ -80,8 +80,12 @@ export function resolveCardDrop(
   try {
     position = generateJitteredKeyBetween(lower, upper);
   } catch {
-    // Identical keys (rare; happens when two rows briefly share a position
+    // Throws whenever `lower >= upper` (the row ordering in `columnRows`
-    // during a concurrent edit). Fall back to insert-after-lower.
+    // briefly diverged from position ordering — typically during a
+    // concurrent reorder). Fall back to insert-after-lower. The card lands
+    // at the end of the column rather than at the dropped slot; surprising
+    // but better than rejecting the drop. The reconciliation arrives via
+    // the realtime patch.
     position = generateJitteredKeyBetween(lower, null);
   }
   return { cells, position };