feat: edge flags, overflow, and add/remove-aware observers

tests/core.test.ts

@@ -29,6 +29,23 @@ class IOShim {
 }
 vi.stubGlobal('IntersectionObserver', IOShim);
 
+// jsdom has no ResizeObserver; record instance so tests can trigger it.
+class ROShim {
+  static last: ROShim | null = null;
+  cb: ResizeObserverCallback;
+  constructor(cb: ResizeObserverCallback) {
+    this.cb = cb;
+    ROShim.last = this;
+  }
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+  fire() {
+    this.cb([] as unknown as ResizeObserverEntry[], this as unknown as ResizeObserver);
+  }
+}
+vi.stubGlobal('ResizeObserver', ROShim);
+
 function makeTrack(n: number): HTMLElement {
   const track = document.createElement('div');
   for (let i = 0; i < n; i++) {
@@ -85,3 +102,76 @@ test('unsubscribe stops delivery; destroy disconnects observer', () => {
   c.destroy();
   expect(spy).toHaveBeenCalled();
 });
+
+test('atStart/atEnd track the current index', () => {
+  const c = createCarousel(track);
+  expect(c.atStart).toBe(true);
+  expect(c.atEnd).toBe(false);
+  (IOShim.last as IOShim).emit(2);
+  expect(c.atStart).toBe(false);
+  expect(c.atEnd).toBe(true);
+});
+
+test('overflow flips on resize and fires change once per change', () => {
+  const c = createCarousel(track);
+  expect(c.overflow).toBe(false);
+  const seen: number[] = [];
+  c.on('change', (i) => seen.push(i));
+
+  Object.defineProperty(track, 'scrollWidth', { value: 100, configurable: true });
+  Object.defineProperty(track, 'clientWidth', { value: 50, configurable: true });
+  (ROShim.last as ROShim).fire();
+  expect(c.overflow).toBe(true);
+  expect(seen).toEqual([0]);
+
+  (ROShim.last as ROShim).fire(); // no change → no re-fire
+  expect(seen).toEqual([0]);
+});
+
+test('adding/removing slides updates count and fires change', async () => {
+  const c = createCarousel(track);
+  const io = IOShim.last as IOShim;
+  const seen: number[] = [];
+  c.on('change', (i) => seen.push(i));
+
+  const observe = vi.spyOn(io, 'observe');
+  const unobserve = vi.spyOn(io, 'unobserve');
+
+  const extra = document.createElement('div');
+  track.append(extra);
+  await Promise.resolve(); // MutationObserver is async (microtask)
+  expect(observe).toHaveBeenCalledWith(extra);
+
+  track.removeChild(extra);
+  await Promise.resolve();
+  expect(unobserve).toHaveBeenCalledWith(extra);
+  expect(c.count).toBe(3);
+  expect(seen.length).toBeGreaterThan(0);
+});
+
+test('overflow recomputes on add/remove without a resize', async () => {
+  Object.defineProperty(track, 'clientWidth', { value: 50, configurable: true });
+  Object.defineProperty(track, 'scrollWidth', { value: 40, configurable: true });
+  const c = createCarousel(track);
+  expect(c.overflow).toBe(false);
+  const seen: number[] = [];
+  c.on('change', (i) => seen.push(i));
+
+  // content now exceeds the track — MutationObserver must pick it up, no RO fire
+  Object.defineProperty(track, 'scrollWidth', { value: 120, configurable: true });
+  track.append(document.createElement('div'));
+  await Promise.resolve(); // MutationObserver is async (microtask)
+  expect(c.overflow).toBe(true);
+  expect(seen.length).toBeGreaterThan(0);
+});
+
+test('removing the current slide clamps index into range', async () => {
+  const c = createCarousel(track);
+  (IOShim.last as IOShim).emit(2);
+  expect(c.index).toBe(2);
+  track.removeChild(track.children[2]);
+  track.removeChild(track.children[1]);
+  await Promise.resolve(); // MutationObserver is async (microtask)
+  expect(c.index).toBe(0);
+  expect(c.atEnd).toBe(true);
+});