Fix logo

frontend/src/hooks/index.ts

@@ -95,6 +95,11 @@ export function usePacedText(text: string, enabled: boolean): string {
   return enabled ? text.slice(0, Math.min(visibleLength, text.length)) : text;
 }
 
+// Each useDarkTheme() caller holds its own state; a toggle broadcasts the new
+// value so every other caller (logo, Mermaid, code blocks) follows without a
+// reload.
+const THEME_CHANGE_EVENT = 'docsgpt:themechange';
+
 export function useDarkTheme() {
   const getSystemThemePreference = () => {
     return (
@@ -126,6 +131,15 @@ export function useDarkTheme() {
     return () => mediaQuery.removeListener(handleChange);
   }, []);
 
+  useEffect(() => {
+    const handleThemeChange = (event: Event) => {
+      setIsDarkTheme((event as CustomEvent<boolean>).detail);
+    };
+    window.addEventListener(THEME_CHANGE_EVENT, handleThemeChange);
+    return () =>
+      window.removeEventListener(THEME_CHANGE_EVENT, handleThemeChange);
+  }, []);
+
   useEffect(() => {
     localStorage.setItem('selectedTheme', isDarkTheme ? 'Dark' : 'Light');
     const action = isDarkTheme ? 'add' : 'remove';
@@ -148,7 +162,11 @@ export function useDarkTheme() {
   }, [isDarkTheme]);
 
   const toggleTheme = () => {
-    setIsDarkTheme(!isDarkTheme);
+    const next = !isDarkTheme;
+    setIsDarkTheme(next);
+    window.dispatchEvent(
+      new CustomEvent<boolean>(THEME_CHANGE_EVENT, { detail: next }),
+    );
   };
 
   return [isDarkTheme, toggleTheme, componentMounted] as const;

frontend/src/hooks/useDarkTheme.test.tsx

@@ -0,0 +1,63 @@
+import { act } from 'react';
+import { createRoot } from 'react-dom/client';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { useDarkTheme } from './index';
+
+Object.assign(globalThis, { IS_REACT_ACT_ENVIRONMENT: true });
+
+describe('useDarkTheme', () => {
+  let container: HTMLDivElement;
+
+  beforeEach(() => {
+    localStorage.setItem('selectedTheme', 'Light');
+    vi.stubGlobal('matchMedia', (query: string) => ({
+      matches: false,
+      media: query,
+      addListener: () => {},
+      removeListener: () => {},
+    }));
+    container = document.createElement('div');
+    document.body.appendChild(container);
+  });
+
+  afterEach(() => {
+    container.remove();
+    localStorage.clear();
+    vi.unstubAllGlobals();
+  });
+
+  it('updates every consumer when one of them toggles the theme', async () => {
+    let toggle: (() => void) | undefined;
+    let logoIsDark: boolean | undefined;
+
+    function Settings() {
+      const [, toggleTheme] = useDarkTheme();
+      toggle = toggleTheme;
+      return null;
+    }
+    function Logo() {
+      [logoIsDark] = useDarkTheme();
+      return null;
+    }
+
+    const root = createRoot(container);
+    await act(async () => {
+      root.render(
+        <>
+          <Settings />
+          <Logo />
+        </>,
+      );
+    });
+    expect(logoIsDark).toBe(false);
+
+    await act(async () => toggle?.());
+    expect(logoIsDark).toBe(true);
+
+    await act(async () => toggle?.());
+    expect(logoIsDark).toBe(false);
+
+    await act(async () => root.unmount());
+  });
+});