From: uazo <uazo@users.noreply.github.com>
Date: Sat, 28 Jun 2025 08:16:28 +0000
Subject: Disable css preferred text scale

Do not expose the user's preferred font scale to CSS.

License: GPL-2.0-or-later - https://spdx.org/licenses/GPL-2.0-or-later.html
---
 .../common/features_cc/Disable-CSSPreferredTextScale.inc      | 1 +
 .../renderer/core/css/document_style_environment_variables.cc | 1 +
 .../blink/renderer/core/css/style_environment_variables.cc    | 4 +++-
 third_party/blink/renderer/core/page/page.cc                  | 4 +++-
 .../blink/renderer/platform/runtime_enabled_features.json5    | 3 +++
 5 files changed, 11 insertions(+), 2 deletions(-)
 create mode 100644 cromite_flags/third_party/blink/common/features_cc/Disable-CSSPreferredTextScale.inc

diff --git a/cromite_flags/third_party/blink/common/features_cc/Disable-CSSPreferredTextScale.inc b/cromite_flags/third_party/blink/common/features_cc/Disable-CSSPreferredTextScale.inc
new file mode 100644
--- /dev/null
+++ b/cromite_flags/third_party/blink/common/features_cc/Disable-CSSPreferredTextScale.inc
@@ -0,0 +1 @@
+SET_CROMITE_FEATURE_DISABLED(kCSSPreferredTextScale);
diff --git a/third_party/blink/renderer/core/css/document_style_environment_variables.cc b/third_party/blink/renderer/core/css/document_style_environment_variables.cc
--- a/third_party/blink/renderer/core/css/document_style_environment_variables.cc
+++ b/third_party/blink/renderer/core/css/document_style_environment_variables.cc
@@ -95,6 +95,7 @@ void DocumentStyleEnvironmentVariables::RecordVariableUsage(
 }
 
 void DocumentStyleEnvironmentVariables::UpdatePreferredTextScaleFromDocument() {
+  if (!RuntimeEnabledFeatures::CSSPreferredTextScaleEnabled()) return;
   double scale_factor;
 
   Settings* settings = document_->GetSettings();
diff --git a/third_party/blink/renderer/core/css/style_environment_variables.cc b/third_party/blink/renderer/core/css/style_environment_variables.cc
--- a/third_party/blink/renderer/core/css/style_environment_variables.cc
+++ b/third_party/blink/renderer/core/css/style_environment_variables.cc
@@ -48,7 +48,9 @@ void SetDefaultEnvironmentVariables(StyleEnvironmentVariables* instance) {
   instance->SetVariable(UADefinedVariable::kKeyboardInsetHeight,
                         kKeyboardInsetDefault);
 
-  instance->SetVariable(UADefinedVariable::kPreferredTextScale, "1");
+  if (RuntimeEnabledFeatures::CSSPreferredTextScaleEnabled()) {
+    instance->SetVariable(UADefinedVariable::kPreferredTextScale, "1");
+  }
 }
 
 }  // namespace.
diff --git a/third_party/blink/renderer/core/page/page.cc b/third_party/blink/renderer/core/page/page.cc
--- a/third_party/blink/renderer/core/page/page.cc
+++ b/third_party/blink/renderer/core/page/page.cc
@@ -1063,7 +1063,9 @@ void Page::SettingsChanged(ChangeType change_type) {
       }
       break;
     case ChangeType::kFontScaleFactor:
-
+      if (!RuntimeEnabledFeatures::CSSPreferredTextScaleEnabled()) {
+        break;
+      }
       for (Frame* frame = MainFrame(); frame;
            frame = frame->Tree().TraverseNext()) {
         LocalFrame* local_frame = DynamicTo<LocalFrame>(frame);
diff --git a/third_party/blink/renderer/platform/runtime_enabled_features.json5 b/third_party/blink/renderer/platform/runtime_enabled_features.json5
--- a/third_party/blink/renderer/platform/runtime_enabled_features.json5
+++ b/third_party/blink/renderer/platform/runtime_enabled_features.json5
@@ -1749,6 +1749,9 @@
       name: "CSSPositionStickyStaticScrollPosition",
       status: "test",
     },
+    {
+      name: "CSSPreferredTextScale", status: "test", public: true
+    },
     // https://drafts.csswg.org/css-values-5/#progress
     // progress()
     {
--