test(miner-ui): cover chat-scroll stick-to-bottom boundary and clamp (#7793)

apps/loopover-miner-ui/src/lib/chat-scroll.ts encodes the 80px near-bottom
threshold and the scroll clamp for the chat auto-scroll feature, but had no
direct test: its only consumer (components/chat/message-list.tsx) is covered
for DOM structure, and jsdom does not meaningfully simulate scrollTop /
scrollHeight there. This logic already needed one dedicated bug-fix pass
(#7229/#7298), which is exactly the kind of regression a unit test catches.

Adds chat-scroll.test.ts following the one-test-per-pure-module convention
demo-data.test.ts already establishes in the same directory:

- isChatViewportNearBottom: the <= boundary at exactly CHAT_NEAR_BOTTOM_PX,
  one pixel past it, sitting exactly at the bottom, content shorter than the
  viewport (negative distance), scrolled far up, and an explicit thresholdPx
  overriding the default.
- scrollChatViewportToBottom: scrolls to the maximum offset, clamps to 0 when
  the content is shorter than the viewport, and is idempotent once bottomed.

Test-only, no production change.

Author: galuis116

apps/loopover-miner-ui/src/lib/chat-scroll.test.ts

@@ -0,0 +1,63 @@
+import { describe, expect, it } from "vitest";
+import { CHAT_NEAR_BOTTOM_PX, isChatViewportNearBottom, scrollChatViewportToBottom } from "./chat-scroll";
+
+/** A minimal stand-in for the scroll viewport: jsdom can't set scrollHeight/clientHeight on a real element. */
+function viewport(scrollHeight: number, clientHeight: number, scrollTop = 0): HTMLElement {
+  return { scrollTop, scrollHeight, clientHeight } as unknown as HTMLElement;
+}
+
+describe("isChatViewportNearBottom (#7229)", () => {
+  it("treats a distance of exactly CHAT_NEAR_BOTTOM_PX as still pinned (the <= boundary)", () => {
+    // scrollHeight - scrollTop - clientHeight === 80
+    expect(isChatViewportNearBottom(viewport(1000, 400, 520))).toBe(true);
+  });
+
+  it("treats one pixel past the threshold as scrolled away", () => {
+    // ...=== 81
+    expect(isChatViewportNearBottom(viewport(1000, 400, 519))).toBe(false);
+  });
+
+  it("is true when the viewport sits exactly at the bottom", () => {
+    expect(isChatViewportNearBottom(viewport(1000, 400, 600))).toBe(true);
+  });
+
+  it("is true for content shorter than the viewport (negative distance, nothing to scroll)", () => {
+    expect(isChatViewportNearBottom(viewport(200, 400, 0))).toBe(true);
+  });
+
+  it("is false when scrolled far up", () => {
+    expect(isChatViewportNearBottom(viewport(5000, 400, 0))).toBe(false);
+  });
+
+  it("honors an explicit thresholdPx instead of the default", () => {
+    // distance === 10: outside a 0px threshold, inside the 80px default.
+    expect(isChatViewportNearBottom(viewport(1000, 400, 590), 0)).toBe(false);
+    expect(isChatViewportNearBottom(viewport(1000, 400, 590))).toBe(true);
+    // distance === 0 still counts at a 0px threshold (<=, not <).
+    expect(isChatViewportNearBottom(viewport(1000, 400, 600), 0)).toBe(true);
+  });
+
+  it("exposes the documented 80px threshold", () => {
+    expect(CHAT_NEAR_BOTTOM_PX).toBe(80);
+  });
+});
+
+describe("scrollChatViewportToBottom (#7229)", () => {
+  it("scrolls to the maximum scrollable offset", () => {
+    const el = viewport(1000, 400, 0);
+    scrollChatViewportToBottom(el);
+    expect(el.scrollTop).toBe(600);
+  });
+
+  it("clamps to 0 when the content is shorter than the viewport", () => {
+    const el = viewport(200, 400, 25);
+    scrollChatViewportToBottom(el);
+    expect(el.scrollTop).toBe(0);
+  });
+
+  it("leaves an already-bottomed viewport at the same offset (idempotent)", () => {
+    const el = viewport(1000, 400, 600);
+    scrollChatViewportToBottom(el);
+    expect(el.scrollTop).toBe(600);
+  });
+});