Bug 1846935 - In helper_relative_scroll_smoothness, relax the expectation on scroll offsets from strictly increasing to non-decreasing for MSD physics. r=hiro

Depends on D195351

Differential Revision: https://phabricator.services.mozilla.com/D195352

gfx/layers/apz/test/mochitest/apz_test_native_event_utils.js

@@ -1971,11 +1971,15 @@ function buildRelativeScrollSmoothnessVariants(aInputType, aScrollMethods) {
   let subtests = [];
   for (let scrollMethod of aScrollMethods) {
     subtests.push({
-      file: `helper_relative_scroll_smoothness.html?input-type=${aInputType}&scroll-method=${scrollMethod}`,
-      prefs: getSmoothScrollPrefs(aInputType, /* Bezier physics */ false)
+      file: `helper_relative_scroll_smoothness.html?input-type=${aInputType}&scroll-method=${scrollMethod}&strict=true`,
+      prefs: getSmoothScrollPrefs(aInputType, /* Bezier physics */ false),
     });
+    // For MSD physics, run the test with strict=false. The shape of the
+    // animation curve is highly timing dependent, and we can't guarantee
+    // that an animation will run long enough until the next input event
+    // arrives.
     subtests.push({
-      file: `helper_relative_scroll_smoothness.html?input-type=${aInputType}&scroll-method=${scrollMethod}`,
+      file: `helper_relative_scroll_smoothness.html?input-type=${aInputType}&scroll-method=${scrollMethod}&strict=false`,
       prefs: getSmoothScrollPrefs(aInputType, /* MSD physics */ true),
     });
   }

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

@@ -31,6 +31,8 @@ body {
 
 <script>
 const searchParams = new URLSearchParams(location.search);
+let strict = searchParams.get("strict") == "true";
+
 var intervalId;
 // Start periodic content expansions after we get a scroll event triggered by
 // a key press in test() function below, otherwise we may have same scroll
@@ -105,8 +107,11 @@ async function test() {
       continue;
     }
     ok(
-      record.scrollOffsetY > previousRecord.scrollOffsetY,
-      "scroll offset should be strictly monotonically increasing " +
+      strict
+        ? (record.scrollOffsetY > previousRecord.scrollOffsetY)
+        : (record.scrollOffsetY >= previousRecord.scrollOffsetY),
+      "scroll offset should be " +
+      (strict ? "strictly monotonically increasing " : "nondecreasing ") +
       "previous offset: " + previousRecord.scrollOffsetY +
       ", offset: " + record.scrollOffsetY
     );