Make flaky snap fling test fail quickly

The flaky test in the linked bug appears to be timing out while waiting
for a scrollend event. This patch modifies the test to listen for
pointer events which signal whether or not a scroll is
happening and allows us to fail/bail early instead of wait for a
timeout.

Bug: 380534434
Change-Id: I2d203cc44b93e8c4c83f9644748916c1d65acbf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6049312
Reviewed-by: Kevin Ellis <[email protected]>
Commit-Queue: David Awogbemila <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1388805}

Author: David Awogbemila

css/css-scroll-snap/snap-fling-in-large-area.html

@@ -44,9 +44,25 @@
         last_scroll_top = scroller.scrollTop;
       }
       scroller.addEventListener("scroll", scroll_listener);
-      const scrollend_promise = waitForScrollendEventNoTimeout(scroller);
-      await touchFlingInTarget(/*pixels_to_scroll*/100, scroller, "down");
-      await scrollend_promise;
+      // We expect pointercancel to be triggered when scrolling starts due to
+      // the touch-fling. If we instead get a pointerup event, we know that
+      // scrolling isn't happening and we can immediately fail the test instead
+      // of waiting for a timeout.
+      const pointer_promise = () => {
+        return new Promise((resolve, reject) => {
+          document.addEventListener("pointerup", reject, { once: true });
+          document.addEventListener("pointercancel", resolve, { once: true });
+        });
+      };
+
+      try {
+        const scrollend_promise = waitForScrollendEventNoTimeout(scroller);
+        await touchFlingInTarget(/*pixels_to_scroll*/100, scroller, "down");
+        await pointer_promise;
+        await scrollend_promise;
+      } catch(e) {
+        assert_unreached("Failed to trigger touch fling.");
+      }
 
       assert_greater_than(last_scroll_top, initial_scroll_top,
         "received at least one scroll update.");