ScrollAnchoring: Limit focus priority to editable elements.

This patch limits the selection of priority candidate to focused
elements with editable style. That is, previously we would consider
any focused element as a priority scroll anchor. Now, this is only
true if the element is editable.

This should be a middle ground that allows most cases in referenced
bug to be fixed (> 30 stars, multiple merged bugs). However, it would
also preserve the layout stability when editing forms with text
input.

R=chrishtr@chromium.org

Fixed: 1102229
Change-Id: I7341f0e513b70f8a4d836cebb68b931f2518534f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363079
Auto-Submit: vmpstr <vmpstr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800259}

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

@@ -11,6 +11,7 @@
 #include "third_party/blink/renderer/core/dom/element_traversal.h"
 #include "third_party/blink/renderer/core/dom/nth_index_cache.h"
 #include "third_party/blink/renderer/core/dom/static_node_list.h"
+#include "third_party/blink/renderer/core/editing/editing_utilities.h"
 #include "third_party/blink/renderer/core/frame/local_frame_view.h"
 #include "third_party/blink/renderer/core/frame/root_frame_viewport.h"
 #include "third_party/blink/renderer/core/frame/web_feature.h"
@@ -338,13 +339,19 @@ bool ScrollAnchor::FindAnchorInPriorityCandidates() {
   auto& document = scroller_box->GetDocument();
 
   // Focused area.
-  LayoutObject* candidate =
-      PriorityCandidateFromNode(document.FocusedElement());
-  auto result = ExaminePriorityCandidate(candidate);
-  if (result.viable) {
-    anchor_object_ = candidate;
-    corner_ = result.corner;
-    return true;
+  LayoutObject* candidate = nullptr;
+  ExamineResult result{kSkip};
+  auto* focused_element = document.FocusedElement();
+  if (focused_element && HasEditableStyle(*focused_element)) {
+    candidate = PriorityCandidateFromNode(focused_element);
+    if (candidate) {
+      result = ExaminePriorityCandidate(candidate);
+      if (result.viable) {
+        anchor_object_ = candidate;
+        corner_ = result.corner;
+        return true;
+      }
+    }
   }
 
   // Active find-in-page match.

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

@@ -1181,7 +1181,7 @@ TEST_F(ScrollAnchorFindInPageTest, FocusPrioritizedOverFindInPage) {
     <div class=spacer></div>
     <div class=spacer></div>
     <div class=spacer></div>
-    <div id=focus_target tabindex=0></div>
+    <div id=focus_target contenteditable></div>
     <div id=growing></div>
     <div id=find_target>findme</div>
     <div class=spacer></div>
@@ -1239,7 +1239,7 @@ TEST_F(ScrollAnchorFindInPageTest, FocusedUnderStickyIsSkipped) {
     <div class=spacer></div>
     <div id=check></div>
     <div class=sticky>
-      <div id=target tabindex=0></div>
+      <div id=target contenteditable></div>
     </div>
     <div class=spacer></div>
     <div class=spacer></div>

third_party/blink/web_tests/external/wpt/css/css-scroll-anchoring/focus-prioritized.html

@@ -21,7 +21,7 @@ body { height: 4000px }
 <div class=spacer></div>
 <div id=growing></div>
 <div class=spacer></div>
-<div id=focused tabindex=0></div>
+<div id=focused contenteditable></div>
 <div class=spacer></div>
 <div class=spacer></div>