Bug 1408122 - Apply prefers-color-scheme: dark to top-level about:blank. r=emilio,bzbarsky

I think during the All Hands in Berlin you might have suggested to do this in nsPresContext::DefaultBackgroundColor,
but this seems a bit more targeted and not a header.

I haven't try tested this yet, so this more of a feedback?

Differential Revision: https://phabricator.services.mozilla.com/D63801

--HG--
extra : moz-landing-system : lando

dom/base/Document.cpp

@@ -16086,6 +16086,37 @@ void Document::RemoveToplevelLoadingDocument(Document* aDoc) {
   }
 }
 
+StylePrefersColorScheme Document::PrefersColorScheme() const {
+  if (nsContentUtils::ShouldResistFingerprinting(this)) {
+    return StylePrefersColorScheme::Light;
+  }
+
+  if (nsPresContext* pc = GetPresContext()) {
+    if (auto devtoolsOverride = pc->GetOverridePrefersColorScheme()) {
+      return *devtoolsOverride;
+    }
+
+    if (pc->IsPrintingOrPrintPreview()) {
+      return StylePrefersColorScheme::Light;
+    }
+  }
+
+  // If LookAndFeel::eIntID_SystemUsesDarkTheme fails then return 2
+  // (no-preference)
+  switch (LookAndFeel::GetInt(LookAndFeel::eIntID_SystemUsesDarkTheme, 2)) {
+    case 0:
+      return StylePrefersColorScheme::Light;
+    case 1:
+      return StylePrefersColorScheme::Dark;
+    case 2:
+      return StylePrefersColorScheme::NoPreference;
+    default:
+      // This only occurs if the user has set the ui.systemUsesDarkTheme pref to
+      // an invalid value.
+      return StylePrefersColorScheme::Light;
+  }
+}
+
 // static
 bool Document::UseOverlayScrollbars(const Document* aDocument) {
   BrowsingContext* bc = aDocument ? aDocument->GetBrowsingContext() : nullptr;

dom/base/Document.h

@@ -152,6 +152,7 @@ class ServoStyleSet;
 enum class StyleOrigin : uint8_t;
 class SMILAnimationController;
 enum class StyleCursorKind : uint8_t;
+enum class StylePrefersColorScheme : uint8_t;
 template <typename>
 class OwningNonNull;
 struct URLExtraData;
@@ -3872,6 +3873,9 @@ class Document : public nsINode,
 
   nsIPermissionDelegateHandler* PermDelegateHandler();
 
+  // CSS prefers-color-scheme media feature for this document.
+  StylePrefersColorScheme PrefersColorScheme() const;
+
   // Returns true if we use overlay scrollbars on the system wide or on the
   // given document.
   static bool UseOverlayScrollbars(const Document* aDocument);

layout/base/PresShell.cpp

@@ -178,6 +178,7 @@
 #include "nsIDragSession.h"
 #include "nsIFrameInlines.h"
 #include "mozilla/gfx/2D.h"
+#include "nsNetUtil.h"
 #include "nsSubDocumentFrame.h"
 #include "nsQueryObject.h"
 #include "mozilla/GlobalStyleSheetCache.h"
@@ -5060,7 +5061,26 @@ nscolor PresShell::GetDefaultBackgroundColorToDraw() {
   if (!mPresContext || !mPresContext->GetBackgroundColorDraw()) {
     return NS_RGB(255, 255, 255);
   }
-  return mPresContext->DefaultBackgroundColor();
+
+  nscolor backgroundColor = mPresContext->DefaultBackgroundColor();
+  if (backgroundColor != NS_RGB(255, 255, 255)) {
+    // Return non-default color.
+    return backgroundColor;
+  }
+
+  // Use a dark background for top-level about:blank that is inaccessible to
+  // content JS.
+  Document* doc = GetDocument();
+  BrowsingContext* bc = doc->GetBrowsingContext();
+  if (bc && bc->IsTop() && !bc->HasOpener() &&
+      doc->GetDocumentURI() &&
+      NS_IsAboutBlank(doc->GetDocumentURI()) &&
+      doc->PrefersColorScheme() == StylePrefersColorScheme::Dark) {
+    // Use --in-content-page-background for prefers-color-scheme: dark.
+    return NS_RGB(0x2A, 0x2A, 0x2E);
+  }
+
+  return backgroundColor;
 }
 
 void PresShell::UpdateCanvasBackground() {

layout/style/nsMediaFeatures.cpp

@@ -242,32 +242,7 @@ bool Gecko_MediaFeatures_PrefersReducedMotion(const Document* aDocument) {
 
 StylePrefersColorScheme Gecko_MediaFeatures_PrefersColorScheme(
     const Document* aDocument) {
-  if (nsContentUtils::ShouldResistFingerprinting(aDocument)) {
-    return StylePrefersColorScheme::Light;
-  }
-  if (nsPresContext* pc = aDocument->GetPresContext()) {
-    if (auto devtoolsOverride = pc->GetOverridePrefersColorScheme()) {
-      return *devtoolsOverride;
-    }
-
-    if (pc->IsPrintingOrPrintPreview()) {
-      return StylePrefersColorScheme::Light;
-    }
-  }
-  // If LookAndFeel::eIntID_SystemUsesDarkTheme fails then return 2
-  // (no-preference)
-  switch (LookAndFeel::GetInt(LookAndFeel::eIntID_SystemUsesDarkTheme, 2)) {
-    case 0:
-      return StylePrefersColorScheme::Light;
-    case 1:
-      return StylePrefersColorScheme::Dark;
-    case 2:
-      return StylePrefersColorScheme::NoPreference;
-    default:
-      // This only occurs if the user has set the ui.systemUsesDarkTheme pref to
-      // an invalid value.
-      return StylePrefersColorScheme::Light;
-  }
+  return aDocument->PrefersColorScheme();
 }
 
 static PointerCapabilities GetPointerCapabilities(const Document* aDocument,

toolkit/themes/shared/in-content/common.inc.css

@@ -107,6 +107,7 @@
 @supports -moz-bool-pref("browser.in-content.dark-mode") {
 @media (prefers-color-scheme: dark) {
   :root {
+    /* Keep this in sync with layout/base/PresShell.cpp! */
     --in-content-page-background: #2A2A2E;
     --in-content-page-color: rgb(249, 249, 250);
     --in-content-text-color: var(--in-content-page-color);