Fix flaky scrolling (#30329)

There are two potential problems here:
1. mouse.scroll returns before the scroll is completed
2. visibility check does not check if the element is actually in the
   viewport.

I've added a helper function to make it easier to scroll to the end of
an infinite list.

playwright/pages/ElementAppPage.ts

@@ -213,4 +213,26 @@ export class ElementAppPage {
             .getByRole("button", { name: "Dismiss" })
             .click();
     }
+
+    /**
+     * Scroll an infinite list to the bottom.
+     * @param list The element to scroll
+     */
+    public async scrollListToBottom(list: Locator): Promise<void> {
+        // First hover the mouse over the element that we want to scroll
+        await list.hover();
+
+        const needsScroll = async () => {
+            // From https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollHeight#determine_if_an_element_has_been_totally_scrolled
+            const fullyScrolled = await list.evaluate(
+                (e) => Math.abs(e.scrollHeight - e.clientHeight - e.scrollTop) <= 1,
+            );
+            return !fullyScrolled;
+        };
+
+        // Scroll the element until we detect that it is fully scrolled
+        do {
+            await this.page.mouse.wheel(0, 1000);
+        } while (await needsScroll());
+    }
 }