diff --git a/changelog.d/added-docs-sidebar-tests.md b/changelog.d/added-docs-sidebar-tests.md
new file mode 100644
index 0000000..dd28564
--- /dev/null
+++ b/changelog.d/added-docs-sidebar-tests.md
@@ -0,0 +1 @@
+- Add behavior tests for the DocsSidebar component (mount-time collapse state, folder and project row toggles, scroll offset throttling).
diff --git a/src/__tests__/DocsSidebar.behavior.test.tsx b/src/__tests__/DocsSidebar.behavior.test.tsx
new file mode 100644
index 0000000..bfb2e57
--- /dev/null
+++ b/src/__tests__/DocsSidebar.behavior.test.tsx
@@ -0,0 +1,474 @@
+// @vitest-environment jsdom
+//
+// Covers the branches of src/components/docs/DocsSidebar.tsx that
+// DocsChrome.behavior.test.tsx leaves untouched:
+//  - mount-time collapse state: the folder chain leading to the active page
+//    stays open (findActivePath), siblings collapse (collapseAll), and
+//    `theme.collapsed === false` pins a folder open
+//  - general sections (Community/Contributing/News): expanded only when the
+//    current path is inside them, with their own nested active-path handling
+//  - getFirstChildRoute(): recursion through nested folders and the
+//    no-navigable-route fallback that renders a plain toggle button
+//  - folder <Link> click expands a collapsed folder without re-collapsing an
+//    open one; the chevron button toggles either way
+//  - hidden items (index, _meta, '#', Separator, Meta, blank title) are not
+//    rendered
+//  - active-project header button collapses/expands the whole tree; the tree
+//    is omitted when the pageMap has no project items
+//  - non-active project rows: label link expands a collapsed row, chevron
+//    toggles, and the docs guide path renders every project as a link
+//  - scroll handling: requestAnimationFrame-throttled offset recalculation
+//  - mobile close button and SidebarFooter collapse wiring through DocsProvider
+import React, { type ReactNode } from "react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import {
+  act,
+  cleanup,
+  fireEvent,
+  render,
+  screen,
+} from "@testing-library/react";
+
+vi.mock("next/link", () => ({
+  default: ({
+    href,
+    children,
+    ...rest
+  }: {
+    href?: string;
+    children?: ReactNode;
+  }) => (
+    <a
+      href={href}
+      {...Object.fromEntries(
+        Object.entries(rest).filter(([key]) => key !== "prefetch")
+      )}
+    >
+      {children}
+    </a>
+  ),
+}));
+
+let pathname = "/docs/hive/overview/introduction";
+vi.mock("next/navigation", () => ({
+  usePathname: () => pathname,
+}));
+
+import { DocsProvider } from "../components/docs/DocsProvider";
+import { DocsSidebar } from "../components/docs/DocsSidebar";
+
+type MenuItem = React.ComponentProps<typeof DocsSidebar>["pageMap"][number];
+
+function renderSidebar(pageMap: MenuItem[], projectId = "hive") {
+  return render(
+    <DocsProvider>
+      <DocsSidebar pageMap={pageMap} projectId={projectId} />
+    </DocsProvider>
+  );
+}
+
+/** The chevron <button> in the same row as the given label text. */
+function chevronFor(label: string): HTMLButtonElement {
+  const row = screen.getByText(label).closest("div");
+  const button = row?.querySelector("button");
+  if (!button) throw new Error(`no chevron button in row for "${label}"`);
+  return button;
+}
+
+/** The collapsible children container that follows the row for `label`. */
+function childrenContainerFor(label: string): HTMLElement {
+  const row = screen.getByText(label).closest("div")!.parentElement!;
+  const container = row.nextElementSibling as HTMLElement | null;
+  if (!container) throw new Error(`no children container after "${label}"`);
+  return container;
+}
+
+const projectTree: MenuItem[] = [
+  { name: "Overview", route: "/docs/hive/overview/introduction" },
+  {
+    name: "Guides",
+    children: [
+      { name: "index", route: "/docs/hive/guides" },
+      { name: "_meta", kind: "Meta" },
+      { name: "Install", route: "/docs/hive/guides/install" },
+      {
+        name: "Advanced",
+        children: [
+          { name: "Tuning", route: "/docs/hive/guides/advanced/tuning" },
+        ],
+      },
+    ],
+  },
+  {
+    name: "Reference",
+    children: [{ name: "CLI", route: "/docs/hive/reference/cli" }],
+  },
+  {
+    name: "Pinned",
+    theme: { collapsed: false },
+    children: [{ name: "Always", route: "/docs/hive/pinned/always" }],
+  },
+  {
+    name: "Community",
+    children: [
+      { name: "Meetings", route: "/docs/community/meetings" },
+      {
+        name: "Events",
+        children: [{ name: "Summit", route: "/docs/community/events/summit" }],
+      },
+    ],
+  },
+  {
+    name: "Contributing",
+    children: [{ name: "Guide", route: "/docs/contributing/guide" }],
+  },
+];
+
+beforeEach(() => {
+  pathname = "/docs/hive/overview/introduction";
+});
+
+afterEach(() => {
+  cleanup();
+  vi.restoreAllMocks();
+  document.body.innerHTML = "";
+});
+
+describe("DocsSidebar initial collapse state", () => {
+  it("opens the folder chain to the active page and collapses the rest", () => {
+    pathname = "/docs/hive/guides/advanced/tuning";
+    renderSidebar(projectTree);
+
+    expect(chevronFor("Guides").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+    expect(chevronFor("Advanced").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+    expect(chevronFor("Reference").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+
+    const active = screen.getByRole("link", { name: "Tuning" });
+    expect(active.className).toContain("text-honey");
+    expect(screen.getByRole("link", { name: "CLI" }).className).not.toContain(
+      "text-honey"
+    );
+  });
+
+  it("keeps theme.collapsed === false folders open when they are not on the active path", () => {
+    renderSidebar(projectTree);
+
+    expect(chevronFor("Pinned").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+    expect(chevronFor("Guides").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+  });
+
+  it("collapses general sections unless the current path is inside one", () => {
+    renderSidebar(projectTree);
+
+    expect(chevronFor("Community").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+    expect(chevronFor("Contributing").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+  });
+
+  it("expands the current general section and its active sub-folder only", () => {
+    pathname = "/docs/community/events/summit";
+    renderSidebar(projectTree);
+
+    expect(chevronFor("Community").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+    expect(chevronFor("Events").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+    expect(chevronFor("Contributing").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+    expect(screen.getByRole("link", { name: "Summit" }).className).toContain(
+      "text-honey"
+    );
+  });
+
+  it("does not render index, _meta, '#', Separator, Meta or untitled items", () => {
+    const pageMap: MenuItem[] = [
+      { name: "index", route: "/docs/hive" },
+      { name: "_meta", kind: "Meta" },
+      { name: "Hidden", route: "#" },
+      { name: "Sep", kind: "Separator" },
+      { name: "", route: "/docs/hive/blank" },
+      { name: "   ", route: "/docs/hive/spaces" },
+      { name: "Visible", route: "/docs/hive/visible" },
+    ];
+    renderSidebar(pageMap);
+
+    const hrefs = screen
+      .getAllByRole("link")
+      .map(link => link.getAttribute("href"));
+    expect(hrefs).toContain("/docs/hive/visible");
+    expect(hrefs).not.toContain("/docs/hive");
+    expect(hrefs).not.toContain("#");
+    expect(hrefs).not.toContain("/docs/hive/blank");
+    expect(hrefs).not.toContain("/docs/hive/spaces");
+    expect(screen.queryByText("Hidden")).toBeNull();
+    expect(screen.queryByText("Sep")).toBeNull();
+  });
+});
+
+describe("DocsSidebar folder rows", () => {
+  it("links a folder to its first navigable route, recursing past Meta items and into nested folders", () => {
+    const pageMap: MenuItem[] = [
+      {
+        name: "Deep",
+        children: [
+          { name: "_meta", kind: "Meta" },
+          { name: "Sep", kind: "Separator" },
+          { name: "Placeholder", route: "#" },
+          {
+            name: "Inner",
+            children: [{ name: "Leaf", route: "/docs/hive/deep/inner/leaf" }],
+          },
+        ],
+      },
+    ];
+    renderSidebar(pageMap);
+
+    expect(
+      screen.getByRole("link", { name: "Deep" }).getAttribute("href")
+    ).toBe("/docs/hive/deep/inner/leaf");
+  });
+
+  it("renders a plain toggle button for a folder with no navigable route", () => {
+    const pageMap: MenuItem[] = [
+      {
+        name: "Empty",
+        children: [
+          { name: "_meta", kind: "Meta" },
+          { name: "Hidden", route: "#" },
+        ],
+      },
+    ];
+    renderSidebar(pageMap);
+
+    expect(screen.queryByRole("link", { name: "Empty" })).toBeNull();
+    const toggle = screen.getByRole("button", { name: "Empty" });
+    const container = toggle.parentElement!.nextElementSibling as HTMLElement;
+    expect(container.className).toContain("max-h-0");
+    fireEvent.click(toggle);
+    expect(container.className).toContain("max-h-[2000px]");
+    fireEvent.click(toggle);
+    expect(container.className).toContain("max-h-0");
+  });
+
+  it("expands a collapsed folder when its label link is clicked but does not collapse an open one", () => {
+    renderSidebar(projectTree);
+
+    const guides = screen.getByRole("link", { name: "Guides" });
+    // The hidden index child is still the folder's first navigable route.
+    expect(guides.getAttribute("href")).toBe("/docs/hive/guides");
+    expect(chevronFor("Guides").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+
+    fireEvent.click(guides);
+    expect(chevronFor("Guides").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+
+    fireEvent.click(guides);
+    expect(chevronFor("Guides").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+  });
+
+  it("toggles a folder from the chevron button regardless of state", () => {
+    renderSidebar(projectTree);
+    const container = childrenContainerFor("Guides");
+
+    expect(container.className).toContain("max-h-0");
+    fireEvent.click(chevronFor("Guides"));
+    expect(container.className).toContain("max-h-[2000px]");
+    fireEvent.click(chevronFor("Guides"));
+    expect(container.className).toContain("max-h-0");
+  });
+});
+
+describe("DocsSidebar project rows", () => {
+  it("collapses and re-expands the active project tree from its header button", () => {
+    renderSidebar(projectTree);
+
+    const header = screen.getByRole("button", { name: "Hive" });
+    const tree = header.nextElementSibling as HTMLElement;
+    expect(tree.className).toContain("max-h-[2000px]");
+
+    fireEvent.click(header);
+    expect(tree.className).toContain("max-h-0");
+    fireEvent.click(header);
+    expect(tree.className).toContain("max-h-[2000px]");
+  });
+
+  it("omits the active project tree when the pageMap has only general sections", () => {
+    renderSidebar([
+      {
+        name: "Community",
+        children: [{ name: "Meetings", route: "/docs/community/meetings" }],
+      },
+    ]);
+
+    expect(screen.queryByRole("button", { name: "Hive" })).toBeNull();
+    expect(screen.getByRole("link", { name: "hotshot" })).toBeTruthy();
+  });
+
+  it("renders non-active projects as links whose chevron reveals an Overview link", () => {
+    renderSidebar(projectTree);
+
+    const pluk = screen.getByRole("link", { name: "pluk" });
+    expect(pluk.getAttribute("href")).toBe("/docs/pluk/overview/introduction");
+
+    const chevron = chevronFor("pluk");
+    const container = pluk.parentElement!.nextElementSibling as HTMLElement;
+    expect(chevron.getAttribute("aria-label")).toBe("Expand section");
+    expect(container.className).toContain("max-h-0");
+
+    fireEvent.click(chevron);
+    expect(chevron.getAttribute("aria-label")).toBe("Collapse section");
+    expect(container.className).toContain("max-h-40");
+    expect(container.querySelector("a")?.getAttribute("href")).toBe(
+      "/docs/pluk/overview/introduction"
+    );
+
+    fireEvent.click(chevron);
+    expect(container.className).toContain("max-h-0");
+  });
+
+  it("expands a collapsed non-active project when its label link is clicked, and leaves an open one alone", () => {
+    renderSidebar(projectTree);
+
+    const dibs = screen.getByRole("link", { name: "dibs" });
+    expect(chevronFor("dibs").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+
+    fireEvent.click(dibs);
+    expect(chevronFor("dibs").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+
+    fireEvent.click(dibs);
+    expect(chevronFor("dibs").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+  });
+
+  it("treats every project as a non-active link on the docs guide page", () => {
+    pathname = "/docs/introduction";
+    renderSidebar(projectTree);
+
+    expect(screen.queryByRole("button", { name: "Hive" })).toBeNull();
+    expect(
+      screen.getByRole("link", { name: "Hive" }).getAttribute("href")
+    ).toBe("/docs/hive/overview/introduction");
+    expect(chevronFor("Hive").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+  });
+
+  it("renders Hive as an expanded link row when projectId is omitted", () => {
+    render(
+      <DocsProvider>
+        <DocsSidebar pageMap={projectTree} />
+      </DocsProvider>
+    );
+
+    // Mount-time collapse treats hive as active (so it is left open), but
+    // rendering compares against the undefined projectId, so no tree is shown.
+    expect(screen.queryByRole("button", { name: "Hive" })).toBeNull();
+    expect(screen.getByRole("link", { name: "Hive" })).toBeTruthy();
+    expect(chevronFor("Hive").getAttribute("aria-label")).toBe(
+      "Collapse section"
+    );
+    expect(chevronFor("pluk").getAttribute("aria-label")).toBe(
+      "Expand section"
+    );
+  });
+});
+
+describe("DocsSidebar layout and chrome", () => {
+  it("throttles scroll-driven offset recalculation through requestAnimationFrame", async () => {
+    document.body.innerHTML = '<div class="nextra-nav-container"></div>';
+    const nav = document.querySelector(".nextra-nav-container") as HTMLElement;
+    let bottom = 64;
+    vi.spyOn(nav, "getBoundingClientRect").mockImplementation(
+      () => ({ bottom }) as DOMRect
+    );
+    const frames: FrameRequestCallback[] = [];
+    vi.spyOn(window, "requestAnimationFrame").mockImplementation(cb => {
+      frames.push(cb);
+      return frames.length;
+    });
+
+    renderSidebar(projectTree);
+    const aside = document.querySelector(
+      'aside[data-sidebar="docs"]'
+    ) as HTMLElement;
+    expect(aside.style.top).toBe("4rem");
+
+    fireEvent.scroll(window);
+    fireEvent.scroll(window);
+    expect(frames).toHaveLength(1);
+
+    bottom = 96;
+    await act(async () => {
+      frames[0](0);
+    });
+    expect(aside.style.top).toBe("96px");
+    expect(aside.style.height).toBe("calc(100vh - 96px)");
+
+    fireEvent.scroll(window);
+    expect(frames).toHaveLength(2);
+  });
+
+  it("leaves layout values alone when no navbar is present", async () => {
+    vi.useFakeTimers();
+    try {
+      renderSidebar(projectTree);
+      const aside = document.querySelector(
+        'aside[data-sidebar="docs"]'
+      ) as HTMLElement;
+      await act(async () => {
+        vi.advanceTimersByTime(400);
+      });
+      expect(aside.style.top).toBe("4rem");
+      expect(aside.style.height).toBe("calc(100vh - 4rem)");
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("closes the mobile menu from the close button and collapses via the footer", () => {
+    renderSidebar(projectTree);
+    const aside = document.querySelector(
+      'aside[data-sidebar="docs"]'
+    ) as HTMLElement;
+    expect(aside.className).toContain("-translate-x-full");
+    expect(aside.className).toContain("lg:w-60");
+
+    // The close button toggles menuOpen; starting closed it opens the drawer.
+    fireEvent.click(screen.getByLabelText("Close sidebar"));
+    expect(aside.className).toContain("translate-x-0 w-60");
+    fireEvent.click(screen.getByLabelText("Close sidebar"));
+    expect(aside.className).toContain("-translate-x-full");
+
+    const collapse = screen.getAllByRole("button", {
+      name: /collapse sidebar/i,
+    })[0];
+    fireEvent.click(collapse);
+    expect(aside.className).toContain("lg:w-16");
+  });
+});