Implement CSS will-change stacking contexts

docs/validation/code-oss-css-will-change-800.md

@@ -0,0 +1,35 @@
+# Code OSS CSS will-change implementation (#800)
+
+## Product trigger
+
+Unchanged Code OSS has 16 `will-change` declarations across 12 CSS files.
+Transform/opacity hints cover Quick Input, Action Widget, sessions/mobile,
+Chat, counters, voice glow, and onboarding. Other hints include
+background-position, box-shadow, left, and top.
+
+## Implemented slice
+
+- Adds generated native/managed identity, bounded validation, and CSSOM for
+  `auto` or up to eight comma-separated ASCII custom identifiers.
+- Retains the normalized authored list in cold textual style and derives one
+  packed hot bit when a hinted non-initial property establishes a stacking
+  context.
+- Reuses the retained atomic stacking-context predicate for paint and hit
+  ordering. It does not preallocate a compositor layer.
+- Treats mutation as paint-only and adds no layout invalidation, visual-tree
+  node, timer, frame participant, platform widget, browser shell, or Code OSS
+  patch.
+
+## Authored gates
+
+- `contracts/css-will-change-stacking-context.html`: CSSOM, invalid writes,
+  mutation/removal, and overlapping descendant/sibling order.
+- `test_css_will_change_stacking_context`: native hit-order mutation contract.
+- `css-will-change-performance`: 4,096 nodes, ten transitions, zero added
+  layout, bounded scene publication, and a five-second guardrail.
+
+## Evidence state
+
+Implementation and gates are authored from source review. Per the active fast
+merge direction, no build, test, WPT, visual, package, memory, lifecycle, or
+performance command was executed. Evidence remains zero until full validation.

experiments/WebScene.NativeEngine.Probe/native/css_property_metadata.json

@@ -156,7 +156,8 @@
     {"id": "overscroll_behavior", "name": "overscroll-behavior", "aliases": ["overscrollbehavior"]},
     {"id": "overscroll_behavior_x", "name": "overscroll-behavior-x", "aliases": ["overscrollbehaviorx"]},
     {"id": "overscroll_behavior_y", "name": "overscroll-behavior-y", "aliases": ["overscrollbehaviory"]},
-    {"id": "isolation", "name": "isolation"}
+    {"id": "isolation", "name": "isolation"},
+    {"id": "will_change", "name": "will-change", "aliases": ["willchange"]}
   ],
   "nativeStorageOnlyProperties": [
     "accent-color", "align-content", "animation-direction", "animation-fill-mode", "animation-play-state",
@@ -178,7 +179,7 @@
       "justify_content", "box_sizing", "vertical_align", "grid_auto_flow", "border_collapse", "table_layout", "border_style", "background_repeat",
       "visibility", "pointer_events", "text_anchor", "font_smoothing", "text_align", "text_transform", "white_space", "word_break", "overflow_wrap", "list_style_position",
       "list_style_type", "scrollbar_width", "container", "container_type", "content_visibility", "object_fit", "user_select",
-      "overscroll_behavior", "overscroll_behavior_x", "overscroll_behavior_y", "isolation"
+      "overscroll_behavior", "overscroll_behavior_x", "overscroll_behavior_y", "isolation", "will_change"
     ],
     "componentList": [
       "container_name", "background_position", "background_size", "transition", "transition_property", "transition_duration",
@@ -222,7 +223,7 @@
     "outline", "outline_width", "outline_color", "animation", "animation_name", "animation_duration", "animation_delay",
     "animation_timing_function", "animation_iteration_count", "background_repeat", "background_position", "background_size",
     "font", "text_transform", "word_break", "overflow_wrap", "list_style", "list_style_position", "list_style_type",
-    "object_fit", "object_position", "user_select", "overscroll_behavior", "overscroll_behavior_x", "overscroll_behavior_y", "isolation"
+    "object_fit", "object_position", "user_select", "overscroll_behavior", "overscroll_behavior_x", "overscroll_behavior_y", "isolation", "will_change"
   ],
   "managedKnownProperties": [
     "align-content", "align-items", "align-self", "all", "background", "background-color",
@@ -244,7 +245,7 @@
     "top", "transform", "visibility", "white-space", "word-break", "overflow-wrap", "width", "word-spacing", "z-index",
     "outline", "outline-color", "outline-offset", "outline-style", "outline-width",
     "grid-template-areas", "aspect-ratio", "object-fit", "object-position", "user-select",
-    "overscroll-behavior", "overscroll-behavior-x", "overscroll-behavior-y", "isolation"
+    "overscroll-behavior", "overscroll-behavior-x", "overscroll-behavior-y", "isolation", "will-change"
   ],
   "supportedPropertyExtras": [
     "animation", "animation-delay", "animation-direction", "animation-duration", "animation-fill-mode",

experiments/WebScene.NativeEngine.Probe/native/generated/webscene_css_property_identity.inc

@@ -232,6 +232,8 @@ inline constexpr std::array native_typed_property_identity_catalog{
     native_typed_property_identity{"overscroll-behavior-y", css_property_id::overscroll_behavior_y},
     native_typed_property_identity{"overscrollbehaviory", css_property_id::overscroll_behavior_y},
     native_typed_property_identity{"isolation", css_property_id::isolation},
+    native_typed_property_identity{"will-change", css_property_id::will_change},
+    native_typed_property_identity{"willchange", css_property_id::will_change},
 };
 
 inline constexpr std::array<std::string_view, 62> native_storage_only_property_catalog{
@@ -456,6 +458,7 @@ inline constexpr std::array native_modeled_property_mask_catalog{
     false,
     false,
     false,
+    false,
 };
 
 inline constexpr bool generated_property_has_modeled_mask(
@@ -659,6 +662,7 @@ inline constexpr std::array native_property_grammar_catalog{
     native_property_grammar::keyword,
     native_property_grammar::keyword,
     native_property_grammar::keyword,
+    native_property_grammar::keyword,
 };
 
 inline constexpr native_property_grammar generated_property_grammar(
@@ -899,5 +903,7 @@ inline css_property_id generated_property_id_lowercase(std::string_view name) no
     if (name == "overscroll-behavior-y") return css_property_id::overscroll_behavior_y;
     if (name == "overscrollbehaviory") return css_property_id::overscroll_behavior_y;
     if (name == "isolation") return css_property_id::isolation;
+    if (name == "will-change") return css_property_id::will_change;
+    if (name == "willchange") return css_property_id::will_change;
     return css_property_id::unknown;
 }

experiments/WebScene.NativeEngine.Probe/native/generated/webscene_css_property_ids.inc

@@ -155,3 +155,4 @@ overscroll_behavior,
 overscroll_behavior_x,
 overscroll_behavior_y,
 isolation,
+will_change,

experiments/WebScene.NativeEngine.Probe/native/generated/webscene_css_supported_properties.inc

@@ -253,11 +253,12 @@ inline constexpr std::array cssom_supported_property_catalog{
     cssom_supported_property_metadata{"white-space", "whiteSpace"},
     cssom_supported_property_metadata{"widows", "widows"},
     cssom_supported_property_metadata{"width", "width"},
+    cssom_supported_property_metadata{"will-change", "willChange"},
     cssom_supported_property_metadata{"word-break", "wordBreak"},
     cssom_supported_property_metadata{"word-spacing", "wordSpacing"},
     cssom_supported_property_metadata{"word-wrap", "wordWrap"},
     cssom_supported_property_metadata{"z-index", "zIndex"},
     cssom_supported_property_metadata{"zoom", "zoom"},
 };
 
-inline constexpr auto cssom_style_template_property_accessor_count = 462U;
+inline constexpr auto cssom_style_template_property_accessor_count = 464U;

experiments/WebScene.NativeEngine.Probe/native/webscene_css_application.h

@@ -261,6 +261,7 @@ void apply_resolved_declaration(native_document& document,dom_node& node,
         } else if (css::apply_user_select_value(node,name,value,decision)) {
         } else if (css::apply_overscroll_behavior_value(node,name,value,decision)) {
         } else if (css::apply_isolation_value(node,name,value,decision)) {
+        } else if (css::apply_will_change_value(node,name,value,decision)) {
         } else if (css::apply_text_value(node,name,value,decision,is_inline)) {
         } else if (name == "border-style") {
             decision.classification = "partially-supported";

experiments/WebScene.NativeEngine.Probe/native/webscene_css_cascade_reset.h

@@ -229,11 +229,13 @@ inline void reset_cascaded_style(dom_node& node,
             textual->object_fit.clear();
             textual->object_position.clear();
             textual->user_select.clear();
+            textual->will_change.clear();
             textual->overscroll_x = overscroll_behavior::automatic;
             textual->overscroll_y = overscroll_behavior::automatic;
         }
         if ((node.style.inline_property_mask & inline_font_weight) == 0U) node.style.font_weight = 0;
         node.style.isolation_stacking_context = false;
+        node.style.will_change_stacking_context = false;
         if ((node.style.inline_property_mask & inline_line_height) == 0U) node.style.line_height = -1;
         if ((node.style.inline_property_mask & inline_letter_spacing) == 0U) {
             node.style.letter_spacing = 0;

experiments/WebScene.NativeEngine.Probe/native/webscene_css_interaction_values.h

@@ -1,5 +1,6 @@
 #pragma once
 #include "webscene_css_property_mask.h"
+#include <cctype>
 #include <sstream>
 #include <utility>
 
@@ -131,4 +132,76 @@ bool apply_isolation_value(dom_node& node,const std::string& name,
     node.style.isolation_stacking_context = value == "isolate";
     return true;
 }
+
+inline std::optional<std::string> normalize_will_change_value(
+    std::string_view raw_value)
+{
+    auto value = ascii_lower(trim_value(raw_value));
+    if (value == "auto") return std::string{};
+    if (value.empty() || value.size() > 256U) return std::nullopt;
+    std::string result;
+    auto start = size_t{0};
+    auto count = size_t{0};
+    while (start <= value.size()) {
+        const auto comma = value.find(',', start);
+        const auto token = trim_value(value.substr(
+            start, comma == std::string::npos ? std::string::npos : comma - start));
+        if (token.empty() || token.size() > 64U || ++count > 8U) return std::nullopt;
+        const auto reserved = token == "auto" || token == "none"
+            || token == "default" || token == "initial" || token == "inherit"
+            || token == "unset" || token == "revert" || token == "revert-layer";
+        const auto identifier = std::all_of(token.begin(), token.end(), [](unsigned char c) {
+            return std::isalnum(c) != 0 || c == '-' || c == '_';
+        }) && (std::isalpha(static_cast<unsigned char>(token.front())) != 0
+            || token.front() == '-' || token.front() == '_');
+        if (reserved || !identifier) return std::nullopt;
+        if (!result.empty()) result += ", ";
+        result += token;
+        if (comma == std::string::npos) break;
+        start = comma + 1U;
+    }
+    return result;
+}
+
+inline bool will_change_establishes_stacking_context(std::string_view value)
+{
+    auto start = size_t{0};
+    while (start <= value.size()) {
+        const auto comma = value.find(',', start);
+        const auto token = trim_value(value.substr(
+            start, comma == std::string::npos ? std::string::npos : comma - start));
+        if (token == "transform" || token == "opacity" || token == "filter"
+            || token == "perspective" || token == "clip-path"
+            || token == "mask" || token == "mask-image"
+            || token == "backdrop-filter") return true;
+        if (comma == std::string::npos) break;
+        start = comma + 1U;
+    }
+    return false;
+}
+
+template<typename Decision>
+bool apply_will_change_value(dom_node& node,const std::string& name,
+    const std::string& raw_value,Decision& decision)
+{
+    if (canonical_property_name(name) != "will-change") return false;
+    auto value = ascii_lower(trim_value(raw_value));
+    if (value == "inherit") {
+        value = node.parent == nullptr
+            ? std::string{} : node.parent->style.textual().will_change;
+    } else if (value == "initial" || value == "unset" || value == "revert"
+        || value == "revert-layer") {
+        value = "auto";
+    }
+    const auto normalized = normalize_will_change_value(value.empty() ? "auto" : value);
+    if (!normalized.has_value()) {
+        decision.classification = "invalid-authoring";
+        return true;
+    }
+    node.style.will_change_stacking_context =
+        will_change_establishes_stacking_context(*normalized);
+    if (normalized->empty() && !node.style.has_textual_data()) return true;
+    node.style.mutable_textual().will_change = *normalized;
+    return true;
+}
 } // namespace webscene_native::css

experiments/WebScene.NativeEngine.Probe/native/webscene_css_reset.h

@@ -251,6 +251,10 @@ inline void apply_all_unset(
     if (has_inline({"isolation"})) {
         reset.isolation_stacking_context = previous.isolation_stacking_context;
     }
+    if (has_inline({"will-change"})) {
+        reset.mutable_textual().will_change = previous.textual().will_change;
+        reset.will_change_stacking_context = previous.will_change_stacking_context;
+    }
 
     // These modeled properties do not yet have dedicated inline-mask
     // bits, so preserve their applied values by authored declaration.

experiments/WebScene.NativeEngine.Probe/native/webscene_css_specified_coverage.h

@@ -11,7 +11,8 @@ inline constexpr auto specified_property_samples = std::to_array<specified_prope
     {"object-fit","cover"},{"object-position","right 25%"},
     {"user-select","none"},
     {"overscroll-behavior","contain none"},{"overscroll-behavior-x","auto"},{"overscroll-behavior-y","contain"},
-    {"isolation","isolate"},
+        {"isolation","isolate"},
+        {"will-change","transform, opacity"},
     {"width","12px"},{"height","20%"},{"min-width","1rem"},{"min-height","2px"},
     {"max-width","none"},{"max-height","fit-content"},
     {"left","1px"},{"top","2px"},{"right","3px"},{"bottom","4px"},{"inset","1px 2px 3px 4px"},

experiments/WebScene.NativeEngine.Probe/native/webscene_native_dom.cpp

@@ -210,6 +210,7 @@ bool style_establishes_atomic_stacking_context(const dom_node& node) noexcept
             && !node.style.z_index_auto)
         || node.style.contain_stacking_context
         || node.style.isolation_stacking_context
+        || node.style.will_change_stacking_context
         || node.style.opacity < 0.999F
         || node.style.transform_stacking_context
         || node.has_painted_filter_override_value()

experiments/WebScene.NativeEngine.Probe/native/webscene_native_dom.h

@@ -802,6 +802,9 @@ struct node_style final {
         // Empty is `auto`; used-value resolution walks ancestors only during
         // the uncommon native text-selection default action.
         std::string user_select;
+        // Retain only authored hints. The hot record stores the derived
+        // stacking-context decision used by every paint/hit traversal.
+        std::string will_change;
         // Scroll-chain policy is consulted only while a wheel default action
         // walks exhausted scroll containers, so keep both axes in cold style.
         overscroll_behavior overscroll_x{overscroll_behavior::automatic};
@@ -1040,6 +1043,7 @@ struct node_style final {
     // CSS isolation is consulted on every retained paint-order walk, so keep
     // it in the existing packed hot flags rather than a token map lookup.
     bool isolation_stacking_context : 1 {false};
+    bool will_change_stacking_context : 1 {false};
     bool content_visibility_hidden : 1 {false};
     bool interpolate_size_allow_keywords : 1 {false};
     // Margin parsing passes these four flags by reference, so unlike the other

experiments/WebScene.NativeEngine.Probe/native/webscene_v8_runtime_dom_core.inc

@@ -743,6 +743,7 @@
             std::string_view{"object-position"},
             std::string_view{"user-select"},
             std::string_view{"isolation"},
+            std::string_view{"will-change"},
             std::string_view{"pointer-events"},
             std::string_view{"visibility"},
             std::string_view{"-webkit-font-smoothing"}

experiments/WebScene.NativeEngine.Probe/native/webscene_v8_runtime_style.inc

@@ -281,6 +281,11 @@
             }
         } else if (name == "isolation") {
             node.style.isolation_stacking_context = false;
+        } else if (name == "willChange" || name == "will-change") {
+            if (auto* textual = node.style.mutable_textual_if_present()) {
+                textual->will_change.clear();
+            }
+            node.style.will_change_stacking_context = false;
         } else if (name == "boxSizing" || name == "box-sizing") {
             node.style.border_box = false;
             node.style.inline_property_mask &= ~inline_box_sizing;
@@ -1001,6 +1006,12 @@
                 std::string semantic_slice;
             } decision;
             css::apply_isolation_value(*node, name, value, decision);
+        } else if (name == "will-change") {
+            struct interaction_decision final {
+                std::string classification{"supported"};
+                std::string semantic_slice;
+            } decision;
+            css::apply_will_change_value(*node, name, value, decision);
         } else if (name == "box-sizing") {
             node->style.border_box = value == "border-box";
             node->style.inline_property_mask |= inline_box_sizing;
@@ -1622,6 +1633,9 @@
                     node->style.textual().overscroll_y);
             } else if (name == "isolation") {
                 value = node->style.isolation_stacking_context ? "isolate" : "auto";
+            } else if (name == "will-change") {
+                value = node->style.textual().will_change.empty()
+                    ? "auto" : node->style.textual().will_change;
             } else if (name == "cursor") {
                 value = resolved_cursor(*node);
             } else if (name == "font-size") {
@@ -2100,6 +2114,9 @@
                 node->style.textual().overscroll_y);
         } else if (name == "isolation") {
             value = node->style.isolation_stacking_context ? "isolate" : "auto";
+        } else if (name == "willChange") {
+            value = node->style.textual().will_change.empty()
+                ? "auto" : node->style.textual().will_change;
         } else if (name == "boxSizing") {
             value = node->style.border_box ? "border-box" : "content-box";
         } else if (name == "borderRadius") {
@@ -2705,6 +2722,12 @@
                 std::string semantic_slice;
             } decision;
             css::apply_isolation_value(*node, name, value, decision);
+        } else if (name == "willChange") {
+            struct interaction_decision final {
+                std::string classification{"supported"};
+                std::string semantic_slice;
+            } decision;
+            css::apply_will_change_value(*node, "will-change", value, decision);
         } else if (name == "boxSizing") {
             node->style.border_box = value == "border-box";
             node->style.inline_property_mask |= inline_box_sizing;

experiments/WebScene.NativeEngine.Probe/native/webscene_v8_runtime_support.inc

@@ -682,6 +682,9 @@ bool valid_cssom_declaration_value(std::string_view property_name, std::string_v
     if (canonical_name == "isolation") {
         return normalized_value == "auto" || normalized_value == "isolate";
     }
+    if (canonical_name == "will-change") {
+        return css::normalize_will_change_value(normalized_value).has_value();
+    }
 
     if (canonical_name == "position") {
         return normalized_value == "static" || normalized_value == "relative"

experiments/WebScene.NativeEngine.Probe/tests/css_property_identity_tests.cpp

@@ -103,10 +103,11 @@ int main()
     static_assert(static_cast<uint16_t>(css_property_id::overscroll_behavior_x) == 151U);
     static_assert(static_cast<uint16_t>(css_property_id::overscroll_behavior_y) == 152U);
     static_assert(static_cast<uint16_t>(css_property_id::isolation) == 153U);
-    static_assert(native_typed_property_identity_catalog.size() == 227U);
+    static_assert(static_cast<uint16_t>(css_property_id::will_change) == 154U);
+    static_assert(native_typed_property_identity_catalog.size() == 229U);
     static_assert(native_storage_only_property_catalog.size() == 62U);
-    static_assert(cssom_supported_property_catalog.size() == 253U);
-    static_assert(cssom_style_template_property_accessor_count == 462U);
+    static_assert(cssom_supported_property_catalog.size() == 254U);
+    static_assert(cssom_style_template_property_accessor_count == 463U);
 
     for (const auto& entry : native_typed_property_identity_catalog) {
         require(property_id(entry.name) == entry.id, "typed name maps to its generated id", entry.name);