diff --git a/e2e/preview-text.test.ts b/e2e/preview-text.test.ts
index c3e60f5..ada9313 100644
--- a/e2e/preview-text.test.ts
+++ b/e2e/preview-text.test.ts
@@ -1,3 +1,4 @@
+import { windowSizeForLine } from '../src/entities/Font/domain/windowSizeForLine/windowSizeForLine';
 import {
     expect,
     test,
@@ -5,12 +6,15 @@ import {
 
 test.describe('preview text', () => {
     test('drives the slider character rendering', async ({ comparison }) => {
+        const text = 'Sphinx';
         await comparison.pickPair('Inter', 'Roboto');
-        await comparison.setPreviewText('Sphinx');
+        await comparison.setPreviewText(text);
 
-        // Window chars render as `.char-wrap` cells for crossfade.
-        // With WINDOW_SIZE=5, "Sphinx" (6 chars) fits 5 in the window.
-        await expect(comparison.slider.locator('.char-wrap')).toHaveCount(5);
+        // Window chars render as `.char-wrap` cells for crossfade. The window
+        // size is a pure function of the line's grapheme count — assert against
+        // the rule, not a hardcoded constant, so tuning the policy can't silently
+        // break this. "Sphinx" is one unwrapped line of 6 graphemes.
+        await expect(comparison.slider.locator('.char-wrap')).toHaveCount(windowSizeForLine(text.length));
     });
 
     test('preserves the typed value in the input', async ({ comparison }) => {