1 files changed, 103 insertions, 0 deletions
diff --git a/src/features/smart-scroll/logic/smart-scroll.test.ts b/src/features/smart-scroll/logic/smart-scroll.test.ts
new file mode 100644
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--- /dev/null
+++ b/src/features/smart-scroll/logic/smart-scroll.test.ts
@@ -0,0 +1,103 @@
+import { describe, expect, it } from "vitest";
+import {
+  createSmartScrollState,
+  isNearBottom,
+  NEAR_BOTTOM_THRESHOLD,
+  onContentChange,
+  onReset,
+  onResume,
+  onScroll,
+  type ScrollGeometry,
+} from "./smart-scroll";
+
+// A viewport 100px tall over 1000px of content: scrollTop 900 == pinned to bottom.
+const atBottom: ScrollGeometry = { scrollTop: 900, scrollHeight: 1000, clientHeight: 100 };
+const nearBottom: ScrollGeometry = {
+  scrollTop: 900 - NEAR_BOTTOM_THRESHOLD,
+  scrollHeight: 1000,
+  clientHeight: 100,
+};
+const scrolledUp: ScrollGeometry = { scrollTop: 200, scrollHeight: 1000, clientHeight: 100 };
+
+describe("isNearBottom", () => {
+  it("is true exactly at the bottom", () => {
+    expect(isNearBottom(atBottom)).toBe(true);
+  });
+
+  it("is true within the threshold of the bottom", () => {
+    expect(isNearBottom(nearBottom)).toBe(true);
+  });
+
+  it("is false just beyond the threshold", () => {
+    expect(
+      isNearBottom({
+        scrollTop: 900 - NEAR_BOTTOM_THRESHOLD - 1,
+        scrollHeight: 1000,
+        clientHeight: 100,
+      }),
+    ).toBe(false);
+  });
+
+  it("is false when scrolled well up", () => {
+    expect(isNearBottom(scrolledUp)).toBe(false);
+  });
+
+  it("honours a custom threshold", () => {
+    const geom: ScrollGeometry = { scrollTop: 800, scrollHeight: 1000, clientHeight: 100 };
+    expect(isNearBottom(geom, 50)).toBe(false);
+    expect(isNearBottom(geom, 150)).toBe(true);
+  });
+});
+
+describe("smart-scroll reducer", () => {
+  it("starts stuck and hides the button", () => {
+    const s = createSmartScrollState();
+    expect(s.stuck).toBe(true);
+  });
+
+  it("onScroll up unsticks and shows the button, with no command", () => {
+    const r = onScroll(createSmartScrollState(), scrolledUp);
+    expect(r.state.stuck).toBe(false);
+    expect(r.showButton).toBe(true);
+    expect(r.command).toBeNull();
+  });
+
+  it("onScroll back to the bottom re-sticks and hides the button", () => {
+    const up = onScroll(createSmartScrollState(), scrolledUp).state;
+    const r = onScroll(up, atBottom);
+    expect(r.state.stuck).toBe(true);
+    expect(r.showButton).toBe(false);
+    expect(r.command).toBeNull();
+  });
+
+  it("onContentChange while stuck emits a NON-animated scroll (keep up with the stream)", () => {
+    const r = onContentChange(createSmartScrollState(), atBottom);
+    expect(r.command).toEqual({ kind: "scroll-to-bottom", animate: false });
+    expect(r.state.stuck).toBe(true);
+  });
+
+  it("onContentChange while unstuck emits NO command (leave the reader in place)", () => {
+    const up = onScroll(createSmartScrollState(), scrolledUp).state;
+    const r = onContentChange(up, scrolledUp);
+    expect(r.command).toBeNull();
+    expect(r.state.stuck).toBe(false);
+    expect(r.showButton).toBe(true);
+  });
+
+  it("onResume re-sticks and emits an ANIMATED scroll", () => {
+    const up = onScroll(createSmartScrollState(), scrolledUp).state;
+    const r = onResume(up);
+    expect(r.state.stuck).toBe(true);
+    expect(r.showButton).toBe(false);
+    expect(r.command).toEqual({ kind: "scroll-to-bottom", animate: true });
+  });
+
+  it("onReset returns to stuck and snaps (non-animated) to the bottom", () => {
+    const up = onScroll(createSmartScrollState(), scrolledUp).state;
+    const r = onReset();
+    void up;
+    expect(r.state.stuck).toBe(true);
+    expect(r.command).toEqual({ kind: "scroll-to-bottom", animate: false });
+    expect(r.showButton).toBe(false);
+  });
+});