diff --git a/src/__tests__/useNavDropdowns.guards.test.tsx b/src/__tests__/useNavDropdowns.guards.test.tsx
new file mode 100644
index 0000000..9cbf890
--- /dev/null
+++ b/src/__tests__/useNavDropdowns.guards.test.tsx
@@ -0,0 +1,230 @@
+// @vitest-environment jsdom
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { act, renderHook } from "@testing-library/react";
+import { useNavDropdowns } from "../components/navbar/useNavDropdowns";
+
+/**
+ * Defensive branches of useNavDropdowns: markup the hook tolerates but the
+ * site never renders on purpose (a [data-dropdown] with no menu next to one
+ * that has one, a menu with no button, a language switcher with no button,
+ * text nodes passing through the MutationObserver). Each case asserts that
+ * the hook neither throws nor flips state it should not.
+ */
+
+function buildDropdown(name: string, { withButton = true } = {}) {
+  const container = document.createElement("div");
+  container.setAttribute("data-dropdown", name);
+
+  let button: HTMLButtonElement | null = null;
+  if (withButton) {
+    button = document.createElement("button");
+    button.setAttribute("data-dropdown-button", "");
+    container.appendChild(button);
+  }
+
+  const menu = document.createElement("div");
+  menu.setAttribute("data-dropdown-menu", "");
+  menu.style.display = "none";
+  container.appendChild(menu);
+
+  document.body.appendChild(container);
+  return { container, button, menu };
+}
+
+function buildBareDropdown(name: string) {
+  const bare = document.createElement("div");
+  bare.setAttribute("data-dropdown", name);
+  document.body.appendChild(bare);
+  return bare;
+}
+
+function buildLangSwitcher({ withButton = true } = {}) {
+  const wrap = document.createElement("div");
+  wrap.className = "language-switcher-container";
+  let button: HTMLButtonElement | null = null;
+  if (withButton) {
+    button = document.createElement("button");
+    wrap.appendChild(button);
+  }
+  document.body.appendChild(wrap);
+  return { wrap, button };
+}
+
+function appendListbox() {
+  const listbox = document.createElement("div");
+  listbox.setAttribute("role", "listbox");
+  document.body.appendChild(listbox);
+  return listbox;
+}
+
+function mouseEnter(el: HTMLElement) {
+  act(() => {
+    el.dispatchEvent(new MouseEvent("mouseenter"));
+  });
+}
+
+function mouseLeave(el: HTMLElement) {
+  act(() => {
+    el.dispatchEvent(new MouseEvent("mouseleave"));
+  });
+}
+
+function advance(ms: number) {
+  act(() => {
+    vi.advanceTimersByTime(ms);
+  });
+}
+
+async function flushObservers() {
+  await act(async () => {
+    await Promise.resolve();
+  });
+}
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  document.body.innerHTML = "";
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+  document.body.innerHTML = "";
+});
+
+describe("useNavDropdowns guard branches", () => {
+  it("opens a dropdown even when a sibling [data-dropdown] has no menu", () => {
+    const { container, menu } = buildDropdown("contribute");
+    buildBareDropdown("community");
+    const { result } = renderHook(() => useNavDropdowns());
+
+    mouseEnter(container);
+    expect(menu.style.display).toBe("block");
+    expect(result.current.isContributeOpen).toBe(true);
+    expect(result.current.isCommunityOpen).toBe(false);
+  });
+
+  it("shows the menu of an unknown dropdown name without naming it in state", () => {
+    const { container, menu } = buildDropdown("docs");
+    const { result } = renderHook(() => useNavDropdowns());
+
+    mouseEnter(container);
+    expect(menu.style.display).toBe("block");
+    expect(result.current).toEqual({
+      isDropdownOpen: true,
+      isContributeOpen: false,
+      isCommunityOpen: false,
+      isGithubOpen: false,
+    });
+  });
+
+  it("works without a [data-dropdown-button]: hover shows, leave hides after the delay", () => {
+    const { container, menu } = buildDropdown("community", {
+      withButton: false,
+    });
+    const { result } = renderHook(() => useNavDropdowns());
+
+    mouseEnter(container);
+    expect(menu.style.display).toBe("block");
+    expect(result.current.isCommunityOpen).toBe(true);
+
+    mouseLeave(container);
+    advance(300);
+    expect(menu.style.display).toBe("none");
+    expect(result.current.isCommunityOpen).toBe(false);
+  });
+
+  it("Escape tolerates a [data-dropdown] without a menu", () => {
+    const { container, menu } = buildDropdown("github");
+    buildBareDropdown("community");
+    const { result } = renderHook(() => useNavDropdowns());
+
+    mouseEnter(container);
+    expect(result.current.isGithubOpen).toBe(true);
+
+    act(() => {
+      document.dispatchEvent(new KeyboardEvent("keydown", { key: "Escape" }));
+    });
+    expect(menu.style.display).toBe("none");
+    expect(result.current.isGithubOpen).toBe(false);
+  });
+
+  describe("language switcher", () => {
+    it("hovering the switcher tolerates a [data-dropdown] without a menu", () => {
+      const { container, menu } = buildDropdown("contribute");
+      buildBareDropdown("community");
+      const { wrap } = buildLangSwitcher();
+      const { result } = renderHook(() => useNavDropdowns());
+
+      mouseEnter(container);
+      expect(menu.style.display).toBe("block");
+
+      mouseEnter(wrap);
+      expect(menu.style.display).toBe("none");
+      expect(result.current.isDropdownOpen).toBe(true);
+    });
+
+    it("does nothing on hover when the switcher has no button", () => {
+      const { wrap } = buildLangSwitcher({ withButton: false });
+      const { result } = renderHook(() => useNavDropdowns());
+
+      mouseEnter(wrap);
+      expect(result.current.isDropdownOpen).toBe(false);
+
+      mouseLeave(wrap);
+      advance(300);
+      expect(result.current.isDropdownOpen).toBe(false);
+    });
+
+    it("leaves a visible listbox alone when the pointer never hovered the switcher", () => {
+      const { wrap, button } = buildLangSwitcher();
+      const click = vi.spyOn(button!, "click");
+      const { result } = renderHook(() => useNavDropdowns());
+
+      // Listbox opened by some other means (keyboard), not via hover.
+      appendListbox();
+      mouseLeave(wrap);
+      advance(300);
+
+      expect(click).not.toHaveBeenCalled();
+      expect(result.current.isDropdownOpen).toBe(false);
+    });
+
+    it("ignores non-element nodes added to or removed from the document", async () => {
+      const { wrap } = buildLangSwitcher();
+      const { result, unmount } = renderHook(() => useNavDropdowns());
+
+      mouseEnter(wrap);
+      expect(result.current.isDropdownOpen).toBe(true);
+
+      const text = document.createTextNode("stray text");
+      act(() => {
+        document.body.appendChild(text);
+      });
+      await flushObservers();
+
+      act(() => {
+        text.remove();
+      });
+      await flushObservers();
+      expect(result.current.isDropdownOpen).toBe(true);
+      unmount();
+    });
+
+    it("entering the listbox with no pending hide is a no-op", async () => {
+      const { wrap, button } = buildLangSwitcher();
+      const click = vi.spyOn(button!, "click");
+      const { result, unmount } = renderHook(() => useNavDropdowns());
+
+      mouseEnter(wrap);
+      const listbox = appendListbox();
+      await flushObservers();
+
+      // No mouseleave has scheduled a hide, so there is nothing to cancel.
+      mouseEnter(listbox);
+      advance(300);
+      expect(click).toHaveBeenCalledTimes(1);
+      expect(result.current.isDropdownOpen).toBe(true);
+      unmount();
+    });
+  });
+});
diff --git a/vitest.config.ts b/vitest.config.ts
index e0ba311..530c446 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -35,12 +35,12 @@ export default defineConfig({
       ],
       exclude: ["**/*.test.ts", "**/*.test.tsx", "**/*.d.ts"],
       // Ratchet: raise these as coverage grows; never lower without a reason
-      // in the PR. Measured at the last ratchet: 99.4 / 99.6 / 94.2 / 98.6.
+      // in the PR. Measured at the last ratchet: 99.7 / 100 / 97.2 / 99.2.
       thresholds: {
-        lines: 97,
-        functions: 97,
-        branches: 92,
-        statements: 96,
+        lines: 98,
+        functions: 98,
+        branches: 95,
+        statements: 97,
       },
     },
   },