Consider scroll-padding when determining scroll anchor node

Scroll anchoring should not anchor to a node that is behind scroll
padding.

Bug: 1010002
Change-Id: Icbd89fb85ea2c97a6de635930a9896f6a87b8f07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887745Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Nick Burris <nburris@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711020}

third_party/blink/renderer/core/layout/scroll_anchor.cc

@@ -292,6 +292,15 @@ ScrollAnchor::ExamineResult ScrollAnchor::Examine(
   LayoutRect visible_rect =
       ScrollerLayoutBox(scroller_)->OverflowClipRect(LayoutPoint());
 
+  const ComputedStyle* style = ScrollerLayoutBox(scroller_)->Style();
+  LayoutRectOutsets scroll_padding(
+      MinimumValueForLength(style->ScrollPaddingTop(), visible_rect.Height()),
+      MinimumValueForLength(style->ScrollPaddingRight(), visible_rect.Width()),
+      MinimumValueForLength(style->ScrollPaddingBottom(),
+                            visible_rect.Height()),
+      MinimumValueForLength(style->ScrollPaddingLeft(), visible_rect.Width()));
+  visible_rect.Contract(scroll_padding);
+
   bool occupies_space =
       candidate_rect.Width() > 0 && candidate_rect.Height() > 0;
   if (occupies_space && visible_rect.Intersects(candidate_rect)) {

third_party/blink/web_tests/external/wpt/css/css-scroll-anchoring/scroll-padding-affects-anchoring-expected.txt

-This is a testharness.js-based test.
-FAIL scroll anchoring accounts for scroll-padding assert_equals: Shouldn't anchor to #changer, since it's covered by scroll-padding expected 150 but got 50
-Harness: the test ran to completion.
-