diff --git a/frontend/src/lib/shared/discussion/MentionEditor.svelte.test.ts b/frontend/src/lib/shared/discussion/MentionEditor.svelte.test.ts
new file mode 100644
index 00000000..e4077e2c
--- /dev/null
+++ b/frontend/src/lib/shared/discussion/MentionEditor.svelte.test.ts
@@ -0,0 +1,225 @@
+import { describe, it, expect, vi, afterEach, beforeEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import MentionEditor from './MentionEditor.svelte';
+
+afterEach(cleanup);
+
+describe('MentionEditor', () => {
+	let fetchSpy: ReturnType<typeof vi.spyOn>;
+
+	beforeEach(() => {
+		fetchSpy = vi.spyOn(globalThis, 'fetch').mockImplementation(async (url: RequestInfo | URL) => {
+			const u = url.toString();
+			if (u.includes('/api/users/search')) {
+				return new Response(
+					JSON.stringify([
+						{ id: 'u1', firstName: 'Anna', lastName: 'Schmidt' },
+						{ id: 'u2', firstName: 'Bertha', lastName: 'Müller' }
+					]),
+					{ status: 200, headers: { 'Content-Type': 'application/json' } }
+				);
+			}
+			return new Response('not found', { status: 404 });
+		});
+	});
+
+	afterEach(() => {
+		fetchSpy?.mockRestore();
+	});
+
+	it('renders the textarea with the placeholder', async () => {
+		render(MentionEditor, {
+			props: {
+				value: '',
+				mentionCandidates: [],
+				placeholder: 'Schreibe etwas…'
+			}
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		expect(ta).not.toBeNull();
+		expect(ta.placeholder).toBe('Schreibe etwas…');
+	});
+
+	it('honours the rows prop', async () => {
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [], rows: 7 }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		expect(ta.rows).toBe(7);
+	});
+
+	it('disables the textarea when disabled is true', async () => {
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [], disabled: true }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		expect(ta.disabled).toBe(true);
+	});
+
+	it('does not show the popup initially', async () => {
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [] }
+		});
+
+		const popup = document.querySelector('[role="listbox"]');
+		expect(popup).toBeNull();
+	});
+
+	it('opens the popup when typing @ followed by a query', async () => {
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [] }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		ta.focus();
+		ta.value = 'Hi @An';
+		ta.selectionStart = 6;
+		ta.selectionEnd = 6;
+		ta.dispatchEvent(new Event('input', { bubbles: true }));
+
+		// debounce 200ms + result render
+		await new Promise((r) => setTimeout(r, 350));
+
+		const popup = document.querySelector('[role="listbox"]');
+		expect(popup).not.toBeNull();
+	});
+
+	it('renders the empty-popup label when fetch returns no results', async () => {
+		fetchSpy.mockImplementationOnce(
+			async () =>
+				new Response('[]', { status: 200, headers: { 'Content-Type': 'application/json' } })
+		);
+
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [] }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		ta.focus();
+		ta.value = '@Zzzz';
+		ta.selectionStart = 5;
+		ta.selectionEnd = 5;
+		ta.dispatchEvent(new Event('input', { bubbles: true }));
+
+		await new Promise((r) => setTimeout(r, 350));
+		await expect.element(page.getByText(/keine nutzer gefunden/i)).toBeVisible();
+	});
+
+	it('clears results when fetch is not OK', async () => {
+		fetchSpy.mockImplementationOnce(async () => new Response('error', { status: 500 }));
+
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [] }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		ta.focus();
+		ta.value = '@Anna';
+		ta.selectionStart = 5;
+		ta.selectionEnd = 5;
+		ta.dispatchEvent(new Event('input', { bubbles: true }));
+
+		await new Promise((r) => setTimeout(r, 350));
+		// Popup is open, but with the empty label
+		await expect.element(page.getByText(/keine nutzer gefunden/i)).toBeVisible();
+	});
+
+	it('calls onsubmit when Enter is pressed without a popup open', async () => {
+		const onsubmit = vi.fn();
+		render(MentionEditor, {
+			props: { value: 'Hello', mentionCandidates: [], onsubmit }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		ta.focus();
+		ta.dispatchEvent(new KeyboardEvent('keydown', { key: 'Enter', bubbles: true }));
+
+		expect(onsubmit).toHaveBeenCalledOnce();
+	});
+
+	it('does not call onsubmit when Shift+Enter is pressed', async () => {
+		const onsubmit = vi.fn();
+		render(MentionEditor, {
+			props: { value: 'Hello', mentionCandidates: [], onsubmit }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		ta.focus();
+		ta.dispatchEvent(new KeyboardEvent('keydown', { key: 'Enter', shiftKey: true, bubbles: true }));
+
+		expect(onsubmit).not.toHaveBeenCalled();
+	});
+
+	it('closes the popup when Escape is pressed', async () => {
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [] }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		ta.focus();
+		ta.value = '@An';
+		ta.selectionStart = 3;
+		ta.selectionEnd = 3;
+		ta.dispatchEvent(new Event('input', { bubbles: true }));
+		await new Promise((r) => setTimeout(r, 350));
+
+		ta.dispatchEvent(new KeyboardEvent('keydown', { key: 'Escape', bubbles: true }));
+		await new Promise((r) => setTimeout(r, 30));
+
+		const popup = document.querySelector('[role="listbox"]');
+		expect(popup).toBeNull();
+	});
+
+	it('navigates results with ArrowDown and ArrowUp', async () => {
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [] }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		ta.focus();
+		ta.value = '@An';
+		ta.selectionStart = 3;
+		ta.selectionEnd = 3;
+		ta.dispatchEvent(new Event('input', { bubbles: true }));
+		await new Promise((r) => setTimeout(r, 350));
+
+		ta.dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowDown', bubbles: true }));
+		await new Promise((r) => setTimeout(r, 30));
+		const opts = document.querySelectorAll('[role="option"]');
+		// Should have moved selection — second item highlighted
+		expect(opts[1]?.getAttribute('aria-selected')).toBe('true');
+
+		ta.dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowUp', bubbles: true }));
+		await new Promise((r) => setTimeout(r, 30));
+		const opts2 = document.querySelectorAll('[role="option"]');
+		expect(opts2[0]?.getAttribute('aria-selected')).toBe('true');
+	});
+
+	it('closes the popup when Enter is hit and no results are present', async () => {
+		fetchSpy.mockImplementationOnce(
+			async () =>
+				new Response('[]', { status: 200, headers: { 'Content-Type': 'application/json' } })
+		);
+		render(MentionEditor, {
+			props: { value: '', mentionCandidates: [] }
+		});
+
+		const ta = document.querySelector('textarea') as HTMLTextAreaElement;
+		ta.focus();
+		ta.value = '@Zz';
+		ta.selectionStart = 3;
+		ta.selectionEnd = 3;
+		ta.dispatchEvent(new Event('input', { bubbles: true }));
+		await new Promise((r) => setTimeout(r, 350));
+
+		ta.dispatchEvent(new KeyboardEvent('keydown', { key: 'Enter', bubbles: true }));
+		await new Promise((r) => setTimeout(r, 30));
+		// Should still be open — Enter without results doesn't close, but doesn't onsubmit either
+		const popup = document.querySelector('[role="listbox"]');
+		expect(popup).not.toBeNull();
+	});
+});