fix(dnd): validate proposed hover destinations

packages/dnd/test/manager-lifecycle.test.ts

@@ -55,6 +55,17 @@ function fixture(overrides: Partial<DndManagerOptions> = {}) {
     instances,
     start: (id: string) => listeners.dragstart!(event(id)),
     end: (id: string) => listeners.dragend!(event(id)),
+    over: (target: { group: string; index: number }) => {
+      let prevented = false;
+      listeners.dragover!({
+        ...event("first"),
+        operation: { ...event("first").operation, target },
+        preventDefault: () => {
+          prevented = true;
+        },
+      });
+      return prevented;
+    },
     get managerDestructions() {
       return managerDestructions;
     },
@@ -82,6 +93,14 @@ test("late drop resolution cannot clear or announce a newer gesture", async () =
   ]);
 });
 
+test("hover acceptance checks the proposed target instead of the source's old group", () => {
+  const f = fixture({ accept: (_ids, to) => to.group === "allowed" });
+  f.start("first");
+  expect(f.over({ group: "blocked", index: 2 })).toBe(true);
+  expect(f.announcements.at(-1)?.location).toEqual({ group: "blocked", index: 2 });
+  expect(f.over({ group: "allowed", index: 0 })).toBe(false);
+});
+
 test("drop commit rejection reports cancellation and its original intent", async () => {
   const failure = new Error("save failed");
   const errors: unknown[] = [];