test(e2e): click only a channel link that is the element hit at its centre

A card inside the viewport can still sit under the chrome above the scroller (the update
banner); the covered click made Playwright scroll and retry, moving the feed the scroll-restore
test then measured. webkit failed 3 in 24 before, 0 in 48 after.

frontend/e2e/helpers.ts

@@ -13,19 +13,24 @@ export async function gotoPage(page: Page, id: string): Promise<void> {
 }
 
 /**
- * Click a channel link that is currently WITHIN the viewport — what a real user does. Never use
- * `.first()` for this after scrolling: the first rendered card lives in the virtualizer's overscan
- * ABOVE the viewport, so Playwright would scroll it into view to click it, moving the scroll
- * position — fatal to any test that then asserts on where the feed was. Returns false if none found.
+ * Click a channel link a real user can actually SEE and hit. Never use `.first()` for this after
+ * scrolling: the first rendered card lives in the virtualizer's overscan ABOVE the viewport, so
+ * Playwright would scroll it into view to click it, moving the scroll position — fatal to any test
+ * that then asserts on where the feed was. Being inside the viewport is not enough either: a card
+ * can sit under the page chrome above the scroller (the update banner, the toolbar band), and the
+ * covered click makes Playwright scroll and retry — the same move (E32 S2: webkit failed 3 in 24).
+ * So the link must be the element hit at its own centre. Returns false if none found.
  */
 export async function clickVisibleChannelLink(page: Page): Promise<boolean> {
   const links = page.getByTestId("video-card-channel").filter({ visible: true });
-  const viewport = page.viewportSize();
-  const height = viewport?.height ?? 800;
   const count = await links.count();
   for (let i = 0; i < count; i++) {
-    const box = await links.nth(i).boundingBox();
-    if (box && box.y >= 0 && box.y + box.height <= height) {
+    const hittable = await links.nth(i).evaluate((el) => {
+      const r = el.getBoundingClientRect();
+      const hit = document.elementFromPoint(r.left + r.width / 2, r.top + r.height / 2);
+      return hit !== null && el.contains(hit);
+    });
+    if (hittable) {
       await links.nth(i).click();
       return true;
     }