[inert] Force 'user-select: none' on inert nodes at used-value time

The CSSWG resolved that inert nodes should behave as if they had
'user-select: none', but without changing the computed style.
w3c/csswg-drafts#6685 (comment)

This is done by adding a public ComputedStyle::UsedUserSelect() that
returns EUserSelect::kNone for inert, and privatizing the existing
ComputedStyle::UserSelect() just for friend classes.

This doesn't seem to have much impact to editing since IsSelectable()
was already checking IsInert().

But now accessibility will set kNotUserSelectableStyle=true when inert.

Bug: 692360

TEST=All/DumpAccessibilityTreeTestWithIgnoredNodes.InertAttribute/blink

AX-Relnotes: Inert nodes will be considered non-selectable.
Change-Id: Iffc15fb90976234768721a6e006de9b6defeee55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3460673
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Cr-Commit-Position: refs/heads/main@{#971185}
NOKEYCHECK=True
GitOrigin-RevId: f1944a2f4cd881ac3b985059e440ecb5384735eb

blink/renderer/core/css/css_properties.json5

@@ -5137,6 +5137,7 @@
     {
       name: "user-select",
       property_methods: ["CSSValueFromComputedStyleInternal"],
+      computed_style_custom_functions: ["getter"],
       inherited: true,
       field_group: "*",
       field_template: "keyword",

blink/renderer/core/dom/node.cc

@@ -1670,13 +1670,13 @@ bool Node::CanStartSelection() const {
 
   if (GetLayoutObject()) {
     const ComputedStyle& style = GetLayoutObject()->StyleRef();
-    if (style.UserSelect() == EUserSelect::kNone)
+    EUserSelect user_select = style.UsedUserSelect();
+    if (user_select == EUserSelect::kNone)
       return false;
     // We allow selections to begin within |user-select: text/all| sub trees
     // but not if the element is draggable.
     if (style.UserDrag() != EUserDrag::kElement &&
-        (style.UserSelect() == EUserSelect::kText ||
-         style.UserSelect() == EUserSelect::kAll))
+        (user_select == EUserSelect::kText || user_select == EUserSelect::kAll))
       return true;
   }
   ContainerNode* parent = FlatTreeTraversal::Parent(*this);

blink/renderer/core/editing/editing_strategy.cc

@@ -22,7 +22,7 @@ blink::EUserSelect UsedValueOfUserSelect(const blink::Node& node) {
   if (style->UserModify() != blink::EUserModify::kReadOnly)
     return blink::EUserSelect::kText;
 
-  return style->UserSelect();
+  return style->UsedUserSelect();
 }
 
 }  // namespace

blink/renderer/core/editing/visible_units_paragraph.cc

@@ -41,7 +41,8 @@ namespace {
 
 bool NodeIsUserSelectAll(const Node* node) {
   return node && node->GetLayoutObject() &&
-         node->GetLayoutObject()->Style()->UserSelect() == EUserSelect::kAll;
+         node->GetLayoutObject()->Style()->UsedUserSelect() ==
+             EUserSelect::kAll;
 }
 
 template <typename Strategy>

blink/renderer/core/frame/smart_clip.cc

@@ -236,7 +236,7 @@ String SmartClip::ExtractTextFromNode(Node* node) {
   StringBuilder result;
   for (Node& current_node : NodeTraversal::InclusiveDescendantsOf(*node)) {
     const ComputedStyle* style = current_node.GetComputedStyle();
-    if (!style || style->UserSelect() == EUserSelect::kNone)
+    if (!style || style->UsedUserSelect() == EUserSelect::kNone)
       continue;
 
     if (Node* node_from_frame = NodeInsideFrame(&current_node))

blink/renderer/core/style/computed_style.h

@@ -143,6 +143,7 @@ class StopColor;
 class Stroke;
 class TextDecorationColor;
 class TextEmphasisColor;
+class UserSelect;
 class WebkitTapHighlightColor;
 class WebkitTextFillColor;
 class WebkitTextStrokeColor;
@@ -257,6 +258,7 @@ class ComputedStyle : public ComputedStyleBase,
   friend class css_longhand::Stroke;
   friend class css_longhand::TextDecorationColor;
   friend class css_longhand::TextEmphasisColor;
+  friend class css_longhand::UserSelect;
   friend class css_longhand::WebkitTapHighlightColor;
   friend class css_longhand::WebkitTextFillColor;
   friend class css_longhand::WebkitTextStrokeColor;
@@ -2241,8 +2243,15 @@ class ComputedStyle : public ComputedStyleBase,
     return PointerEventsInternal();
   }
 
+  // User select utility functions.
+  EUserSelect UsedUserSelect() const {
+    if (IsInert())
+      return EUserSelect::kNone;
+    return UserSelectInternal();
+  }
+
   bool IsSelectable() const {
-    return !IsInert() && !(UserSelect() == EUserSelect::kNone &&
+    return !IsInert() && !(UserSelectInternal() == EUserSelect::kNone &&
                            UserModify() == EUserModify::kReadOnly);
   }
 
@@ -2785,6 +2794,7 @@ class ComputedStyle : public ComputedStyleBase,
   EFloat Floating() const { return FloatingInternal(); }
   EPointerEvents PointerEvents() const { return PointerEventsInternal(); }
   EResize Resize() const { return ResizeInternal(); }
+  EUserSelect UserSelect() const { return UserSelectInternal(); }
 
   bool IsInlineSizeContainer() const {
     return ContainerType() & kContainerTypeInlineSize;

blink/renderer/modules/accessibility/ax_layout_object.cc

@@ -377,7 +377,7 @@ bool AXLayoutObject::IsNotUserSelectable() const {
   if (!style)
     return false;
 
-  return (style->UserSelect() == EUserSelect::kNone);
+  return (style->UsedUserSelect() == EUserSelect::kNone);
 }
 
 //