Scroll anchoring: use correct corner when restoring in vertical-rl writing mode

Bug: 1111201
Change-Id: Iac335d42a332e4a6bee7bcda5c588c9d74de679a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2331070Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793676}

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

@@ -597,9 +597,13 @@ bool ScrollAnchor::RestoreAnchor(const SerializedAnchor& serialized_anchor) {
     // and attempt to re-find the anchor. The user-visible effect should end up
     // roughly the same.
     ScrollOffset current_offset = scroller_->GetScrollOffset();
-    FloatPoint desired_point =
-        anchor_object->AbsoluteBoundingBoxFloatRect().Location() +
-        current_offset;
+    FloatRect bounding_box = anchor_object->AbsoluteBoundingBoxFloatRect();
+    FloatPoint location_point =
+        anchor_object->Style()->IsFlippedBlocksWritingMode()
+            ? bounding_box.MaxXMinYCorner()
+            : bounding_box.Location();
+    FloatPoint desired_point = location_point + current_offset;
+
     ScrollOffset desired_offset =
         ScrollOffset(desired_point.X(), desired_point.Y());
     ScrollOffset delta =
@@ -647,6 +651,7 @@ const SerializedAnchor ScrollAnchor::GetSerializedAnchor() {
   SerializedAnchor new_anchor(
       ComputeUniqueSelector(anchor_object_->GetNode()),
       ComputeRelativeOffset(anchor_object_, scroller_, corner_));
+
   if (new_anchor.IsValid()) {
     saved_selector_ = new_anchor.selector;
   }

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

@@ -751,6 +751,30 @@ TEST_P(ScrollAnchorTest, SerializeAnchorVerticalWritingMode) {
   ValidateSerializedAnchor("html>body>.barbaz", LayoutPoint(-50, 0));
 }
 
+TEST_P(ScrollAnchorTest, RestoreAnchorVerticalRlWritingMode) {
+  SetBodyInnerHTML(R"HTML(
+      <style>
+      body {
+          height: 100px;
+          margin: 0;
+          writing-mode:
+          vertical-rl;
+        }
+        div.big { width: 800px; }
+        div { width: 100px; height: 100px; }
+      </style>
+      <div class='big'></div>
+      <div id='last'></div>
+      )HTML");
+
+  SerializedAnchor serialized_anchor("#last", LayoutPoint(0, 0));
+
+  EXPECT_TRUE(
+      GetScrollAnchor(LayoutViewport()).RestoreAnchor(serialized_anchor));
+  EXPECT_EQ(LayoutViewport()->ScrollOffsetInt().Width(), 0);
+  EXPECT_EQ(LayoutViewport()->ScrollOffsetInt().Height(), 0);
+}
+
 TEST_P(ScrollAnchorTest, SerializeAnchorQualifiedTagName) {
   SetBodyInnerHTML(R"HTML(
       <style>