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feat(nav): add ThemeToggle component with moon/sun icons and no-flash script

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- Inline <script> in app.html applies saved localStorage theme before first
  paint to prevent flash of wrong theme
- ThemeToggle.svelte: moon/sun button, localStorage persistence, sets
  data-theme on <html>, defaults to system preference on first visit
- Placed in +layout.svelte between language selector and user menu
- E2E tests cover visibility, toggle, reverse toggle, persistence, and
  no-flash behaviour — all 6 passing

Refs #64
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
Marcel
2026-03-25 11:57:09 +01:00
parent e83ba9b681
commit caba89dacc
4 changed files with 152 additions and 0 deletions
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73
frontend/e2e/theme.spec.ts Normal file
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@@ -0,0 +1,73 @@
import { test, expect } from '@playwright/test';
test.describe('Theme toggle', () => {
test.beforeEach(async ({ page }) => {
// Clear any saved theme preference before each test
await page.goto('/');
await page.evaluate(() => localStorage.removeItem('theme'));
});
test('toggle button is visible in the header', async ({ page }) => {
await page.goto('/');
await expect(
page.getByRole('banner').getByRole('button', { name: /dark mode|light mode/i })
).toBeVisible();
});
test('clicking the toggle switches to dark mode', async ({ page }) => {
await page.goto('/');
await page.waitForSelector('[data-hydrated]');
const html = page.locator('html');
await expect(html).not.toHaveAttribute('data-theme', 'dark');
await page
.getByRole('banner')
.getByRole('button', { name: /dark mode/i })
.click();
await expect(html).toHaveAttribute('data-theme', 'dark');
});
test('clicking the toggle again switches back to light mode', async ({ page }) => {
await page.goto('/');
await page.waitForSelector('[data-hydrated]');
await page
.getByRole('banner')
.getByRole('button', { name: /dark mode/i })
.click();
await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
await page
.getByRole('banner')
.getByRole('button', { name: /light mode/i })
.click();
await expect(page.locator('html')).toHaveAttribute('data-theme', 'light');
});
test('theme persists after page reload', async ({ page }) => {
await page.goto('/');
await page.waitForSelector('[data-hydrated]');
await page
.getByRole('banner')
.getByRole('button', { name: /dark mode/i })
.click();
await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
await page.reload();
await expect(page.locator('html')).toHaveAttribute('data-theme', 'dark');
});
test('saved theme is applied before first paint (no flash)', async ({ page }) => {
// Set dark theme in localStorage before navigating
await page.goto('/');
await page.evaluate(() => localStorage.setItem('theme', 'dark'));
// Intercept the initial HTML to verify data-theme is set immediately
await page.goto('/');
const theme = await page.evaluate(() => document.documentElement.getAttribute('data-theme'));
expect(theme).toBe('dark');
});
});

6
frontend/src/app.html
View File

@@ -3,6 +3,12 @@
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<script>
(function () {
var t = localStorage.getItem('theme');
if (t === 'dark' || t === 'light') document.documentElement.setAttribute('data-theme', t);
})();
</script>
%sveltekit.head%
</head>
<body data-sveltekit-preload-data="hover">

69
frontend/src/lib/components/ThemeToggle.svelte Normal file
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@@ -0,0 +1,69 @@
<script lang="ts">
import { onMount } from 'svelte';
type Theme = 'light' | 'dark';
function systemPrefersDark(): boolean {
return window.matchMedia('(prefers-color-scheme: dark)').matches;
}
function resolveInitialTheme(): Theme {
const saved = localStorage.getItem('theme');
if (saved === 'light' || saved === 'dark') return saved;
return systemPrefersDark() ? 'dark' : 'light';
}
let theme = $state<Theme>('light');
onMount(() => {
theme = resolveInitialTheme();
});
function toggle() {
theme = theme === 'dark' ? 'light' : 'dark';
localStorage.setItem('theme', theme);
document.documentElement.setAttribute('data-theme', theme);
}
</script>
<button
type="button"
onclick={toggle}
aria-label={theme === 'dark' ? 'light mode' : 'dark mode'}
title={theme === 'dark' ? 'light mode' : 'dark mode'}
class="rounded p-1.5 text-ink-2 transition-colors hover:bg-muted hover:text-ink"
>
{#if theme === 'dark'}
<!-- Sun icon — click to go light -->
<svg
class="h-4 w-4"
viewBox="0 0 24 24"
fill="none"
stroke="currentColor"
stroke-width="2"
aria-hidden="true"
>
<circle cx="12" cy="12" r="4" />
<path
stroke-linecap="round"
d="M12 2v2M12 20v2M4.22 4.22l1.42 1.42M18.36 18.36l1.42 1.42M2 12h2M20 12h2M4.22 19.78l1.42-1.42M18.36 5.64l1.42-1.42"
/>
</svg>
{:else}
<!-- Moon icon — click to go dark -->
<svg
class="h-4 w-4"
viewBox="0 0 24 24"
fill="none"
stroke="currentColor"
stroke-width="2"
aria-hidden="true"
>
<path
stroke-linecap="round"
stroke-linejoin="round"
d="M21 12.79A9 9 0 1111.21 3 7 7 0 0021 12.79z"
/>
</svg>
{/if}
</button>

4
frontend/src/routes/+layout.svelte
View File

@@ -5,6 +5,7 @@ import { page } from '$app/state';
import { onMount } from 'svelte';
import { m } from '$lib/paraglide/messages.js';
import { setLocale, getLocale } from '$lib/paraglide/runtime';
import ThemeToggle from '$lib/components/ThemeToggle.svelte';
let { children, data } = $props();
@@ -125,6 +126,9 @@ function clickOutside(node: HTMLElement) {
{/each}
</div>
<!-- Theme toggle -->
<ThemeToggle />
<!-- User menu -->
<div
class="relative"