Make @media (prefers-color-scheme:) respect features::kWebUIDarkMode

while on chrome:// URLs

This allows us to use this mechanism in CSS:

  @media (prefers-color-scheme: dark) {
  }

to vary styles for Chrome web UI in dark mode while also disabling via
feature on certain platforms.

This unblocks us from migrating to the @media query (from the existing
[dark] attribute syntax) and later removing a bit of JS/C++ custom code
in favor of simply using the web platform.

R=futhark@chromium.org
BUG=965811

Change-Id: I79f4582ce21661d7ba2f6a33f3f054f5ff62d0db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626024
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667245}

chrome/browser/chrome_content_browser_client.cc

@@ -155,6 +155,7 @@
 #include "chrome/browser/ui/prefs/pref_watcher.h"
 #include "chrome/browser/ui/sync/sync_promo_ui.h"
 #include "chrome/browser/ui/tab_contents/chrome_web_contents_view_delegate.h"
+#include "chrome/browser/ui/ui_features.h"
 #include "chrome/browser/ui/webui/chrome_web_ui_controller_factory.h"
 #include "chrome/browser/ui/webui/log_web_ui_url.h"
 #include "chrome/browser/usb/usb_tab_helper.h"
@@ -3425,7 +3426,7 @@ void ChromeContentBrowserClient::OverrideWebkitPrefs(
     }
   }
 
-  auto* native_theme = ui::NativeTheme::GetInstanceForWeb();
+  auto* native_theme = GetWebTheme();
 #if !defined(OS_ANDROID)
   if (IsAutoplayAllowedByPolicy(contents, prefs)) {
     // If autoplay is allowed by policy then force the no user gesture required
@@ -3450,6 +3451,15 @@ void ChromeContentBrowserClient::OverrideWebkitPrefs(
 
   web_prefs->translate_service_available = TranslateService::IsAvailable(prefs);
 
+  // Force a light preferred color scheme on chrome:// pages if kWebUIDarkMode
+  // is disabled until all UI is correctly themed and OSes support dark mode.
+  // Note: the WebUI CSS explicitly uses light (instead of not dark), which is
+  // why we don't reset back to no-preference. https://crbug.com/965811
+  if (contents && contents->GetURL().SchemeIs(content::kChromeUIScheme) &&
+      !base::FeatureList::IsEnabled(features::kWebUIDarkMode)) {
+    web_prefs->preferred_color_scheme = blink::PreferredColorScheme::kLight;
+  }
+
   // Apply native CaptionStyle parameters.
   // TODO(ellyjones): Support more native caption styling.
   ui::CaptionStyle style = native_theme->GetSystemCaptionStyle();
@@ -5301,6 +5311,10 @@ bool ChromeContentBrowserClient::HandleWebUIReverse(
          url->host() == chrome::kChromeUISettingsHost;
 }
 
+const ui::NativeTheme* ChromeContentBrowserClient::GetWebTheme() const {
+  return ui::NativeTheme::GetInstanceForWeb();
+}
+
 // static
 void ChromeContentBrowserClient::SetDefaultQuotaSettingsForTesting(
     const storage::QuotaSettings* settings) {

chrome/browser/chrome_content_browser_client.h

@@ -65,6 +65,14 @@ class SafeBrowsingService;
 class UrlCheckerDelegate;
 }
 
+namespace ui {
+class NativeTheme;
+}
+
+namespace url {
+class Origin;
+}
+
 namespace user_prefs {
 class PrefRegistrySyncable;
 }
@@ -73,10 +81,6 @@ namespace version_info {
 enum class Channel;
 }
 
-namespace url {
-class Origin;
-}
-
 class ChromeHidDelegate;
 class ChromeSerialDelegate;
 
@@ -637,6 +641,7 @@ class ChromeContentBrowserClient : public content::ContentBrowserClient {
   static bool HandleWebUI(GURL* url, content::BrowserContext* browser_context);
   static bool HandleWebUIReverse(GURL* url,
                                  content::BrowserContext* browser_context);
+  virtual const ui::NativeTheme* GetWebTheme() const;  // For testing.
 
  private:
   friend class DisableWebRtcEncryptionFlagTest;

chrome/browser/chrome_content_browser_client_browsertest.cc

@@ -4,6 +4,7 @@
 
 #include "chrome/browser/chrome_content_browser_client.h"
 
+#include <memory>
 #include <vector>
 
 #include "base/base_switches.h"
@@ -23,6 +24,7 @@
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/search/instant_test_base.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
+#include "chrome/browser/ui/ui_features.h"
 #include "chrome/common/chrome_features.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/pref_names.h"
@@ -36,8 +38,10 @@
 #include "content/public/browser/navigation_entry.h"
 #include "content/public/browser/render_frame_host.h"
 #include "content/public/browser/render_process_host.h"
+#include "content/public/browser/render_view_host.h"
 #include "content/public/browser/site_isolation_policy.h"
 #include "content/public/browser/web_contents.h"
+#include "content/public/common/content_client.h"
 #include "content/public/common/content_features.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/test/browser_test_utils.h"
@@ -49,6 +53,8 @@
 #include "net/test/embedded_test_server/http_request.h"
 #include "net/test/embedded_test_server/http_response.h"
 #include "testing/gmock/include/gmock/gmock.h"
+#include "ui/native_theme/native_theme.h"
+#include "ui/native_theme/test_native_theme.h"
 #include "url/gurl.h"
 #include "url/origin.h"
 
@@ -545,22 +551,57 @@ IN_PROC_BROWSER_TEST_F(OpenWindowFromNTPBrowserTest,
 class PrefersColorSchemeTest : public testing::WithParamInterface<bool>,
                                public InProcessBrowserTest {
  protected:
-  PrefersColorSchemeTest() = default;
+  PrefersColorSchemeTest() : theme_client_(&test_theme_) {}
+
+  ~PrefersColorSchemeTest() {
+    CHECK_EQ(&theme_client_, SetBrowserClientForTesting(original_client_));
+  }
+
   const char* ExpectedColorScheme() const {
-    return ui::NativeTheme::GetInstanceForNativeUi()->SystemDarkModeEnabled()
-               ? "dark"
-               : "light";
+    return GetParam() ? "dark" : "light";
   }
+
   void SetUpCommandLine(base::CommandLine* command_line) override {
     InProcessBrowserTest::SetUpCommandLine(command_line);
-    command_line->AppendSwitchASCII("enable-blink-features",
+    command_line->AppendSwitchASCII(switches::kEnableBlinkFeatures,
                                     "MediaQueryPrefersColorScheme");
-    if (GetParam())
-      command_line->AppendSwitch("force-dark-mode");
   }
+
+  void SetUpOnMainThread() override {
+    InProcessBrowserTest::SetUpOnMainThread();
+    original_client_ = SetBrowserClientForTesting(&theme_client_);
+  }
+
+ protected:
+  ui::TestNativeTheme test_theme_;
+
+ private:
+  content::ContentBrowserClient* original_client_ = nullptr;
+
+  class ChromeContentBrowserClientWithWebTheme
+      : public ChromeContentBrowserClient {
+   public:
+    explicit ChromeContentBrowserClientWithWebTheme(
+        const ui::NativeTheme* theme)
+        : theme_(theme) {}
+
+   protected:
+    const ui::NativeTheme* GetWebTheme() const override { return theme_; }
+
+   private:
+    const ui::NativeTheme* const theme_;
+  };
+
+  ChromeContentBrowserClientWithWebTheme theme_client_;
 };
 
 IN_PROC_BROWSER_TEST_P(PrefersColorSchemeTest, PrefersColorScheme) {
+  test_theme_.SetDarkMode(GetParam());
+  browser()
+      ->tab_strip_model()
+      ->GetActiveWebContents()
+      ->GetRenderViewHost()
+      ->OnWebkitPreferencesChanged();
   ui_test_utils::NavigateToURL(
       browser(),
       ui_test_utils::GetTestUrl(
@@ -571,6 +612,23 @@ IN_PROC_BROWSER_TEST_P(PrefersColorSchemeTest, PrefersColorScheme) {
   EXPECT_EQ(base::ASCIIToUTF16(ExpectedColorScheme()), tab_title);
 }
 
+IN_PROC_BROWSER_TEST_P(PrefersColorSchemeTest, WebUIFeatureOverrides) {
+  test_theme_.SetDarkMode(true);
+
+  base::test::ScopedFeatureList features;
+  features.InitWithFeatureState(features::kWebUIDarkMode, GetParam());
+
+  ui_test_utils::NavigateToURL(browser(), GURL(chrome::kChromeUIDownloadsURL));
+
+  bool in_dark_mode;  // A default shouldn't matter because of the ASSERT().
+  ASSERT_TRUE(ExecuteScriptAndExtractBool(
+      browser()->tab_strip_model()->GetActiveWebContents(),
+      "window.domAutomationController.send("
+      "    window.matchMedia('(prefers-color-scheme: dark)').matches)",
+      &in_dark_mode));
+  EXPECT_EQ(in_dark_mode, GetParam());
+}
+
 INSTANTIATE_TEST_SUITE_P(All, PrefersColorSchemeTest, testing::Bool());
 
 class ProtocolHandlerTest : public InProcessBrowserTest {

chrome/browser/chrome_content_browser_client_unittest.cc

@@ -429,7 +429,7 @@ TEST(ChromeContentBrowserClientTest, GetMetricSuffixForURL) {
                     GURL("https://www.google.com/search?notaquery=nope")));
 }
 
-TEST(ChromeContentBrowserClient, UserAgentStringFrozen) {
+TEST(ChromeContentBrowserClientTest, UserAgentStringFrozen) {
   base::test::ScopedFeatureList scoped_feature_list;
   scoped_feature_list.InitAndEnableFeature(blink::features::kFreezeUserAgent);
 
@@ -468,7 +468,7 @@ TEST(ChromeContentBrowserClient, UserAgentStringFrozen) {
 #endif
 }
 
-TEST(ChromeContentBrowserClient, UserAgentStringOrdering) {
+TEST(ChromeContentBrowserClientTest, UserAgentStringOrdering) {
 #if defined(OS_ANDROID)
   const char* const kArguments[] = {"chrome"};
   base::test::ScopedCommandLine scoped_command_line;
@@ -488,7 +488,7 @@ TEST(ChromeContentBrowserClient, UserAgentStringOrdering) {
 #endif
 }
 
-TEST(ChromeContentBrowserClient, UserAgentMetadata) {
+TEST(ChromeContentBrowserClientTest, UserAgentMetadata) {
   ChromeContentBrowserClient content_browser_client;
   auto metadata = content_browser_client.GetUserAgentMetadata();
 

chrome/browser/ui/OWNERS

@@ -21,6 +21,9 @@ per-file browser_instant_controller*=file://chrome/browser/search/OWNERS
 per-file avatar_*=file://components/signin/OWNERS
 per-file signin_*=file://components/signin/OWNERS
 
+per-file ui_features.cc=*
+per-file ui_features.h=*
+
 per-file settings_window_manager*chromeos*=file://chrome/browser/ui/ash/OWNERS
 
 # COMPONENT: UI>Browser

chrome/browser/ui/ui_features.cc

@@ -39,7 +39,6 @@ const char kTabHoverCardsFeatureParameterName[] = "setting";
 const base::Feature kTabHoverCardImages{"TabHoverCardImages",
                                         base::FEATURE_DISABLED_BY_DEFAULT};
 
-#if !defined(OS_ANDROID)
 // Whether to enable "dark mode" enhancements in Mac Mojave or Windows 10 for
 // UIs implemented with web technologies.
 const base::Feature kWebUIDarkMode {
@@ -50,6 +49,5 @@ const base::Feature kWebUIDarkMode {
       base::FEATURE_DISABLED_BY_DEFAULT
 #endif  // defined(OS_MACOSX) || defined(OS_WIN)
 };
-#endif  // !defined(OS_ANDROID)
 
 }  // namespace features

chrome/browser/ui/ui_features.h

@@ -31,9 +31,7 @@ extern const char kTabHoverCardsFeatureParameterName[];
 
 extern const base::Feature kTabHoverCardImages;
 
-#if !defined(OS_ANDROID)
 extern const base::Feature kWebUIDarkMode;
-#endif
 
 }  // namespace features