test(discussion): expand MentionEditor coverage further

Adds mousedown-to-select flow, Enter-to-select with highlighted
result, fetch network throw handling.

3 new tests covering ~6 branches.

Refs #496.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/MentionEditor.svelte.test.ts

@@ -222,4 +222,71 @@ describe('MentionEditor', () => {
 		const popup = document.querySelector('[role="listbox"]');
 		expect(popup).not.toBeNull();
 	});
+
+	it('selects a user via mousedown click and fills the textarea', 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));
+
+		const firstOption = document.querySelector('[role="option"]') as HTMLElement;
+		firstOption?.dispatchEvent(new MouseEvent('mousedown', { bubbles: true }));
+
+		await new Promise((r) => setTimeout(r, 50));
+		// After select, popup is closed
+		const popup = document.querySelector('[role="listbox"]');
+		expect(popup).toBeNull();
+	});
+
+	it('selects via Enter when results are present', 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));
+
+		// Move highlight to first result with ArrowDown then Enter
+		ta.dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowDown', bubbles: true }));
+		await new Promise((r) => setTimeout(r, 30));
+		ta.dispatchEvent(new KeyboardEvent('keydown', { key: 'Enter', bubbles: true }));
+		await new Promise((r) => setTimeout(r, 50));
+
+		// Popup closed after select
+		const popup = document.querySelector('[role="listbox"]');
+		expect(popup).toBeNull();
+	});
+
+	it('handles fetch network throw gracefully', async () => {
+		fetchSpy.mockImplementationOnce(async () => {
+			throw new Error('network down');
+		});
+
+		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));
+		// Empty popup label after thrown fetch
+		await expect.element(page.getByText(/keine nutzer gefunden/i)).toBeVisible();
+	});
 });