fix: make observe cleanup idempotent

src/__tests__/observe.test.ts

@@ -19,6 +19,32 @@ test("should be able to use observe", () => {
   );
 });
 
+test("should only clean up each observer callback once", () => {
+  const element = document.createElement("div");
+  const firstCallback = vi.fn();
+  const secondCallback = vi.fn();
+  const firstCleanup = observe(element, firstCallback, { threshold: 0.1 });
+  const secondCleanup = observe(element, secondCallback, { threshold: 0.1 });
+  const observer = intersectionMockInstance(element);
+
+  firstCleanup();
+  firstCleanup();
+  mockIsIntersecting(element, true);
+
+  expect(firstCallback).not.toHaveBeenCalled();
+  expect(secondCallback).toHaveBeenCalledTimes(1);
+  expect(observer.unobserve).not.toHaveBeenCalled();
+
+  secondCleanup();
+  expect(observer.unobserve).toHaveBeenCalledTimes(1);
+  expect(observer.unobserve).toHaveBeenCalledWith(element);
+  expect(observer.disconnect).toHaveBeenCalledTimes(1);
+
+  secondCleanup();
+  expect(observer.unobserve).toHaveBeenCalledTimes(1);
+  expect(observer.disconnect).toHaveBeenCalledTimes(1);
+});
+
 test("should convert options to id", () => {
   expect(
     optionsToId({

src/observe.ts

@@ -156,7 +156,11 @@ export function observe(
   callbacks.push(callback);
   observer.observe(element);
 
+  let unobserved = false;
   return function unobserve() {
+    if (unobserved) return;
+    unobserved = true;
+
     // Remove the callback from the callback list
     callbacks.splice(callbacks.indexOf(callback), 1);