1 files changed, 172 insertions, 0 deletions
diff --git a/src/features/smart-scroll/ui/controller.test.ts b/src/features/smart-scroll/ui/controller.test.ts
new file mode 100644
index 0000000..614f4b0
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+++ b/src/features/smart-scroll/ui/controller.test.ts
@@ -0,0 +1,172 @@
+import { describe, expect, it, vi } from "vitest";
+import { createSmartScrollController } from "./controller.svelte";
+
+// A minimal fake of the only DOM surface the controller touches: scroll
+// geometry, scrollTo, and add/removeEventListener for "scroll"/"scrollend".
+// Faking this outermost edge is the sanctioned mock (no internal modules mocked).
+function createFakeScrollEl(opts?: { scrollHeight?: number; clientHeight?: number }) {
+	const listeners = new Map<string, Set<EventListener>>();
+	const el = {
+		scrollTop: 0,
+		scrollHeight: opts?.scrollHeight ?? 1000,
+		clientHeight: opts?.clientHeight ?? 100,
+		scrollTo: vi.fn((arg: ScrollToOptions) => {
+			// Emulate the browser: jump scrollTop, then (for "instant") fire scrollend.
+			el.scrollTop = (arg.top ?? 0) - 0;
+			if (arg.behavior !== "smooth") {
+				fire("scroll");
+				fire("scrollend");
+			}
+		}),
+		addEventListener: (type: string, fn: EventListener) => {
+			if (!listeners.has(type)) listeners.set(type, new Set());
+			listeners.get(type)?.add(fn);
+		},
+		removeEventListener: (type: string, fn: EventListener) => {
+			listeners.get(type)?.delete(fn);
+		},
+	};
+	function fire(type: string): void {
+		for (const fn of listeners.get(type) ?? []) fn(new Event(type));
+	}
+	// Simulate the USER scrolling to a given offset (fires scroll, not self-driven).
+	function userScrollTo(top: number): void {
+		el.scrollTop = top;
+		fire("scroll");
+	}
+	return {
+		el: el as unknown as HTMLElement,
+		scrollTo: el.scrollTo,
+		fire,
+		userScrollTo,
+		listenerCount: () => listeners,
+	};
+}
+
+describe("smart-scroll controller", () => {
+	it("starts with the button hidden", () => {
+		const c = createSmartScrollController();
+		expect(c.showButton).toBe(false);
+	});
+
+	it("contentChanged while stuck scrolls to the bottom instantly", () => {
+		const c = createSmartScrollController();
+		const fake = createFakeScrollEl();
+		c.attach(fake.el);
+		c.contentChanged();
+		expect(fake.scrollTo).toHaveBeenCalledWith({
+			top: 1000,
+			behavior: "instant",
+		});
+		expect(c.showButton).toBe(false);
+	});
+
+	it("a user scroll up shows the button and stops auto-following", () => {
+		const c = createSmartScrollController();
+		const fake = createFakeScrollEl();
+		c.attach(fake.el);
+		fake.userScrollTo(200); // far from the bottom
+		expect(c.showButton).toBe(true);
+
+		const scrollTo = fake.scrollTo;
+		scrollTo.mockClear();
+		c.contentChanged(); // streaming more content...
+		expect(scrollTo).not.toHaveBeenCalled(); // ...must NOT yank the reader down
+		expect(c.showButton).toBe(true);
+	});
+
+	it("self-driven scrolls are not misread as the user scrolling up", () => {
+		const c = createSmartScrollController();
+		const fake = createFakeScrollEl();
+		c.attach(fake.el);
+		// contentChanged drives an instant scrollTo, whose synthetic scroll event
+		// must NOT flip us to unstuck (selfScrolling guard).
+		c.contentChanged();
+		expect(c.showButton).toBe(false);
+	});
+
+	it("resume re-sticks and smooth-scrolls to the bottom", () => {
+		const c = createSmartScrollController();
+		const fake = createFakeScrollEl();
+		c.attach(fake.el);
+		fake.userScrollTo(200);
+		expect(c.showButton).toBe(true);
+
+		c.resume();
+		expect(fake.scrollTo).toHaveBeenCalledWith({
+			top: 1000,
+			behavior: "smooth",
+		});
+		expect(c.showButton).toBe(false);
+	});
+
+	it("reset snaps to the bottom and hides the button", () => {
+		const c = createSmartScrollController();
+		const fake = createFakeScrollEl();
+		c.attach(fake.el);
+		fake.userScrollTo(200);
+		expect(c.showButton).toBe(true);
+		c.reset();
+		expect(fake.scrollTo).toHaveBeenCalledWith({
+			top: 1000,
+			behavior: "instant",
+		});
+		expect(c.showButton).toBe(false);
+	});
+
+	it("observes content via a ResizeObserver: follows growth while stuck, not while unstuck", () => {
+		const holder: { cb: ResizeObserverCallback | null } = { cb: null };
+		const observed: unknown[] = [];
+		const disconnect = vi.fn();
+		class FakeResizeObserver {
+			constructor(cb: ResizeObserverCallback) {
+				holder.cb = cb;
+			}
+			observe(target: Element): void {
+				observed.push(target);
+			}
+			unobserve(): void {}
+			disconnect = disconnect;
+		}
+		vi.stubGlobal("ResizeObserver", FakeResizeObserver);
+		try {
+			const c = createSmartScrollController();
+			const fake = createFakeScrollEl();
+			const content = { id: "content" } as unknown as HTMLElement;
+			const teardown = c.attach(fake.el, content);
+
+			// Observes both the content (it grows) and the scroll container (viewport resize).
+			expect(observed).toContain(content);
+			expect(observed).toContain(fake.el);
+
+			// Stuck → a resize keeps us pinned to the bottom.
+			fake.scrollTo.mockClear();
+			holder.cb?.([], {} as ResizeObserver);
+			expect(fake.scrollTo).toHaveBeenCalledWith({ top: 1000, behavior: "instant" });
+
+			// Reader scrolls up → a later resize must NOT yank them down.
+			fake.userScrollTo(200);
+			fake.scrollTo.mockClear();
+			holder.cb?.([], {} as ResizeObserver);
+			expect(fake.scrollTo).not.toHaveBeenCalled();
+
+			// Teardown disconnects the observer.
+			teardown();
+			expect(disconnect).toHaveBeenCalled();
+		} finally {
+			vi.unstubAllGlobals();
+		}
+	});
+
+	it("attach returns a teardown that removes both listeners", () => {
+		const c = createSmartScrollController();
+		const fake = createFakeScrollEl();
+		const teardown = c.attach(fake.el);
+		const before = fake.listenerCount();
+		expect(before.get("scroll")?.size).toBe(1);
+		expect(before.get("scrollend")?.size).toBe(1);
+		teardown();
+		expect(before.get("scroll")?.size).toBe(0);
+		expect(before.get("scrollend")?.size).toBe(0);
+	});
+});