Bug 1774537 - Skip re-snapping if there's any async scroll is running. r=botond

Depends on D149497

Differential Revision: https://phabricator.services.mozilla.com/D150929

gfx/layers/apz/test/mochitest/helper_scroll_snap_resnap_after_async_scroll.html

@@ -8,6 +8,9 @@
   <script src="apz_test_native_event_utils.js"></script>
   <script src="/tests/SimpleTest/paint_listener.js"></script>
   <style>
+    body {
+      margin: 0;
+    }
     div {
       position: absolute;
     }
@@ -15,7 +18,7 @@
       width: 100%;
       height: 500px;
       overflow-y: scroll;
-      scroll-snap-type: y mandatory;
+      scroll-snap-type: y proximity; /* to escape from the last snap point */
     }
     .child {
       width: 100%;
@@ -29,10 +32,13 @@
   <div id="scroller">
     <div class="child" style="top: 0px;"></div>
     <div class="child" style="top: 200px;"></div>
-    <div style="width: 100%; height: 2000px;"></div>
+    <div id="spacer" style="width: 100%; height: 2000px;"></div>
   </div>
   <script type="application/javascript">
     async function test() {
+      const proximityThreshold =
+          SpecialPowers.getIntPref("layout.css.scroll-snap.proximity-threshold");
+
       let transformEndPromise = promiseTransformEnd();
       await promiseMoveMouseAndScrollWheelOver(scroller, 100, 100);
 
@@ -43,6 +49,28 @@
 
       document.querySelectorAll(".child")[1].style.top = "400px";
       is(scroller.scrollTop, 400, "re-snap to 400px");
+
+      // Make sure the new snap position has been reflected in APZ side.
+      await promiseApzFlushedRepaints();
+
+      // Now trigger another wheel scroll to escape from the last snap point.
+      transformEndPromise = promiseTransformEnd();
+      // With a small `layout.css.scroll-behavior.spring-constant` preference
+      // value (it's set in test_group_scroll_snap.html), an async scroll
+      // operation, e.g. a wheel scroll operation, fires multiple scroll events
+      // so that here we wait the first scroll event here by specifying
+      // `waitForScroll=true`, there should be other scroll events after the
+      // first one.
+      await promiseMoveMouseAndScrollWheelOver(scroller, 100, 100,
+          true /* waitForScroll */, proximityThreshold + 1000);
+
+      // Expand the scrollable region during the wheel scroll.
+      spacer.style.height = "2200px";
+
+      await transformEndPromise;
+      await promiseApzFlushedRepaints();
+
+      isnot(scroller.scrollTop, 400, "Do not re-snap to the original 400px point");
     }
 
     waitUntilApzStable()

gfx/layers/apz/test/mochitest/helper_scroll_snap_resnap_after_async_scrollBy.html

@@ -8,6 +8,9 @@
   <script src="apz_test_native_event_utils.js"></script>
   <script src="/tests/SimpleTest/paint_listener.js"></script>
   <style>
+    body {
+      margin: 0;
+    }
     div {
       position: absolute;
     }
@@ -15,7 +18,7 @@
       width: 100%;
       height: 500px;
       overflow-y: scroll;
-      scroll-snap-type: y mandatory;
+      scroll-snap-type: y proximity; /* to escape from the last snap point */
     }
     .child {
       width: 100%;
@@ -29,10 +32,13 @@
   <div id="scroller">
     <div class="child" style="top: 0px;"></div>
     <div class="child" style="top: 200px;"></div>
-    <div style="width: 100%; height: 2000px;"></div>
+    <div id="spacer" style="width: 100%; height: 2000px;"></div>
   </div>
   <script type="application/javascript">
     async function test() {
+      const proximityThreshold =
+          SpecialPowers.getIntPref("layout.css.scroll-snap.proximity-threshold");
+
       let transformEndPromise = promiseTransformEnd();
       scroller.scrollBy({left: 0, top: 100, behavior: "smooth"});
 
@@ -43,6 +49,19 @@
 
       document.querySelectorAll(".child")[1].style.top = "400px";
       is(scroller.scrollTop, 400, "re-snap to 400px");
+
+      // Now trigger another async scroll to escape from the last snap point.
+      transformEndPromise = promiseTransformEnd();
+      scroller.scrollBy({left: 0, top: proximityThreshold + 100,
+                         behavior: "smooth"});
+
+      // Expand the scrollable region during the async scroll.
+      spacer.style.height = "2200px";
+
+      await transformEndPromise;
+      await promiseApzFlushedRepaints();
+
+      isnot(scroller.scrollTop, 400, "Do not re-snap to the original 400px point");
     }
 
     waitUntilApzStable()

gfx/layers/apz/test/mochitest/test_group_scroll_snap.html

@@ -19,10 +19,18 @@ const prefs = [
 
 const subtests = [
   {"file": "helper_scroll_snap_no_valid_snap_position.html", "prefs": prefs},
-  {"file": "helper_scroll_snap_resnap_after_async_scroll.html"},
   {"file": "helper_scroll_snap_resnap_after_async_scroll.html",
-    "prefs": [["general.smoothScroll", false]]},
-  {"file": "helper_scroll_snap_resnap_after_async_scrollBy.html"},
+    // Specify a small `layout.css.scroll-behavior.spring-constant` value to
+    // keep the smooth scroll animation running long enough so that we can
+    // trigger a reflow during the animation.
+   "prefs": [["layout.css.scroll-behavior.spring-constant", 10],
+             ["apz.test.mac.synth_wheel_input", true]]},
+  {"file": "helper_scroll_snap_resnap_after_async_scroll.html",
+    "prefs": [["general.smoothScroll", false],
+             ["apz.test.mac.synth_wheel_input", true]]},
+  {"file": "helper_scroll_snap_resnap_after_async_scrollBy.html",
+    // Same as above helper_scroll_snap_resnap_after_async_scroll.html.
+   "prefs": [["layout.css.scroll-behavior.spring-constant", 10]]},
 ];
 
 if (isApzEnabled()) {

layout/generic/nsGfxScrollFrame.cpp

@@ -8041,6 +8041,11 @@ bool ScrollFrameHelper::IsLastSnappedTarget(const nsIFrame* aFrame) const {
 }
 
 void ScrollFrameHelper::TryResnap() {
+  // If there's any async scroll is running, don't clobber the scroll.
+  if (!ScrollAnimationState().isEmpty()) {
+    return;
+  }
+
   if (auto snapTarget = GetSnapPointForResnap()) {
     // We are going to re-snap so that we need to clobber scroll anchoring.
     mAnchor.UserScrolled();