Bug 1833972 [wpt PR 40083] - [scroll-start] Apply scroll-start after layout, a=testonly

Automatic update from web-platform-tests
[scroll-start] Apply scroll-start after layout

scroll-start[1] should set the initial/default scroll position of a
scroller. Many different things may cause a scroller's size/layout
to change as a page loads which can affect how scroll-start should
be applied. So, this patch applies scroll-start on each frame until
the scroller has been explicitly scrolled, i.e. by the user or via a
programmatic scroll. After an explicit scroll, scroll-start is no
longer applied.

We propagate scroll-start from the document element to the viewport
so that scroll-start can be applied to the page. We save an
explicitly_scrolled_ bit on the element so that scroll-start
does not prevent saved (explicit) scrolls from surviving changes
to display and overflow.

[1] https://drafts.csswg.org/css-scroll-snap-2/#scroll-start

Bug: 1439807
Change-Id: I5655f8e74b85fe45038f700091065fa11eeac0f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4546420
Commit-Queue: David Awogbemila <awogbemila@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1159106}

--

wpt-commits: 4203f37b7415c420aa495d315339e8e5b51b8288
wpt-pr: 40083

testing/web-platform/tests/css/css-scroll-snap-2/scroll-start-fieldset.html

@@ -41,6 +41,10 @@
   </fieldset>
   <script>
     let scroller = document.getElementById("scroller");
+    // fieldsets' clientHeight and scrollHeight can be affected by the presence of
+    // a scrollbar which has been anecdotally measured to be 15 on several
+    // platforms.
+    const scrollbar_width = 15;
     const max_vertical_scroll_offset = scroller.scrollHeight -
       scroller.clientHeight;
     // The fieldset's width is set based on the size of its contents:
@@ -56,7 +60,6 @@
         expectation: {
           scrollTop: 100,
           msg: "scroll-start: <length> sets initial scroll position",
-          approx: false
         }
       },
       {
@@ -64,7 +67,6 @@
         expectation: {
           scrollTop: 0.25 * max_vertical_scroll_offset,
           msg: "scroll-start: <percent> sets initial scroll position",
-          approx: true
         }
       },
       {
@@ -72,7 +74,6 @@
         expectation: {
           scrollTop: 50,
           msg: "scroll-start: <calc> sets initial scroll position",
-          approx: false
         }
       },
       {
@@ -80,7 +81,6 @@
         expectation: {
           scrollTop: 0,
           msg: "scroll-start: start sets initial scroll position",
-          approx: false
         }
       },
       {
@@ -88,7 +88,6 @@
         expectation: {
           scrollTop: 0.5 * max_vertical_scroll_offset,
           msg: "scroll-start: center sets initial scroll position",
-          approx: true
         }
       },
       {
@@ -96,7 +95,6 @@
         expectation: {
           scrollTop: max_vertical_scroll_offset,
           msg: "scroll-start: end sets initial scroll position",
-          approx: false
         }
       },
       {
@@ -104,7 +102,6 @@
         expectation: {
           scrollTop: 0,
           msg: "scroll-start: top sets initial scroll position",
-          approx: false
         }
       },
       {
@@ -112,7 +109,6 @@
         expectation: {
           scrollTop: max_vertical_scroll_offset,
           msg: "scroll-start: bottom sets initial scroll position",
-          approx: false
         }
       },
       {
@@ -120,7 +116,6 @@
         expectation: {
           scrollTop: max_vertical_scroll_offset,
           msg: "scroll-start is clamped",
-          approx: false
         }
       }
     ];
@@ -139,13 +134,11 @@
     async function test_scroll_start(scroll_start, expectation) {
       await resetScroller(scroll_start);
 
-      if (expectation.approx) {
-        assert_approx_equals(scroller.scrollTop, expectation.scrollTop, 1, expectation.msg);
-      } else {
-        assert_equals(scroller.scrollTop, expectation.scrollTop, expectation.msg);
-      }
+      assert_approx_equals(scroller.scrollTop, expectation.scrollTop,
+        scrollbar_width, expectation.msg);
     }
 
+
     promise_test(async () => {
       for (let test_case of test_cases) {
         await test_scroll_start(test_case.scroll_start,