Issue #1752 - Implement "prefers-color-scheme" as a user preference.

This PR passes all current tests for this feature, and implements the "prefers-color-scheme" media query as an enumerated keyword that is controlled by an integer preference.

Currently, the possible options are 0 to see a website's fallback code and essentially behave like this isn't implemented (our current behavior), 1 to express a preference for a light theme (the default for spec reasons), and 2 to express a preference for a dark theme. Over time, this list may expand to include other preferences like a preference for a sepia theme or something, and this leaves us prepared for that future.

3 files changed, 40 insertions, 0 deletions

diff --git a/layout/style/nsCSSKeywordList.h b/layout/style/nsCSSKeywordList.h
index 689ff47eb6..be1691e629 100644
--- a/layout/style/nsCSSKeywordList.h
+++ b/layout/style/nsCSSKeywordList.h
@@ -224,6 +224,7 @@ CSS_KEY(crosshair, crosshair)
 CSS_KEY(currentcolor, currentcolor)
 CSS_KEY(cursive, cursive)
 CSS_KEY(cyclic, cyclic)
+CSS_KEY(dark, dark)
 CSS_KEY(darken, darken)
 CSS_KEY(dashed, dashed)
 CSS_KEY(dense, dense)
@@ -364,6 +365,7 @@ CSS_KEY(last baseline, last_baseline) // only used for DevTools auto-completion
 CSS_KEY(layout, layout)
 CSS_KEY(left, left)
 CSS_KEY(legacy, legacy)
+CSS_KEY(light, light)
 CSS_KEY(lighten, lighten)
 CSS_KEY(lighter, lighter)
 CSS_KEY(line-through, line_through)
diff --git a/layout/style/nsMediaFeatures.cpp b/layout/style/nsMediaFeatures.cpp
index e44003ce48..9b5331a4a1 100644
--- a/layout/style/nsMediaFeatures.cpp
+++ b/layout/style/nsMediaFeatures.cpp
@@ -20,6 +20,7 @@
 #include "nsIDocument.h"
 #include "nsIWidget.h"
 #include "nsContentUtils.h"
+#include "mozilla/Preferences.h"
 #include "mozilla/StyleSheet.h"
 #include "mozilla/StyleSheetInlines.h"
 
@@ -45,6 +46,12 @@ static const nsCSSProps::KTableEntry kDisplayModeKeywords[] = {
   { eCSSKeyword_UNKNOWN,                 -1 }
 };
 
+static const nsCSSProps::KTableEntry kPrefersColorSchemeKeywords[] = {
+  { eCSSKeyword_light,			 NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT },
+  { eCSSKeyword_dark,			 NS_STYLE_PREFERS_COLOR_SCHEME_DARK },
+  { eCSSKeyword_UNKNOWN,		 -1 },
+};
+
 #ifdef XP_WIN
 struct WindowsThemeName {
   LookAndFeel::WindowsTheme id;
@@ -456,6 +463,25 @@ GetOperatingSystemVersion(nsPresContext* aPresContext, const nsMediaFeature* aFe
 }
 
 static nsresult
+GetPrefersColorScheme(nsPresContext* aPresContext, const nsMediaFeature* aFeature,
+		      nsCSSValue& aResult)
+{
+  switch(Preferences::GetInt("browser.display.prefers_color_scheme", 1)) {
+    case 1:
+    aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT, 
+		        eCSSUnit_Enumerated);
+    break;
+    case 2:
+    aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_DARK, 
+		        eCSSUnit_Enumerated);
+    break;
+    default:
+    aResult.Reset();
+  }
+  return NS_OK;
+}
+
+static nsresult
 GetIsGlyph(nsPresContext* aPresContext, const nsMediaFeature* aFeature,
           nsCSSValue& aResult)
 {
@@ -555,6 +581,14 @@ nsMediaFeatures::features[] = {
     GetMonochrome
   },
   {
+    &nsGkAtoms::prefers_color_scheme,
+    nsMediaFeature::eMinMaxNotAllowed,
+    nsMediaFeature::eEnumerated,
+    nsMediaFeature::eNoRequirements,
+    { kPrefersColorSchemeKeywords },
+    GetPrefersColorScheme
+  },
+  {
     &nsGkAtoms::resolution,
     nsMediaFeature::eMinMaxAllowed,
     nsMediaFeature::eResolution,
diff --git a/layout/style/nsStyleConsts.h b/layout/style/nsStyleConsts.h
index 308a66647b..60377928b0 100644
--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -1306,6 +1306,10 @@ enum class StyleDisplay : uint8_t {
 #define NS_STYLE_DISPLAY_MODE_STANDALONE        2
 #define NS_STYLE_DISPLAY_MODE_FULLSCREEN        3
 
+// prefers-color-scheme
+#define NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT	0
+#define NS_STYLE_PREFERS_COLOR_SCHEME_DARK	1
+
 } // namespace mozilla
 
 #endif /* nsStyleConsts_h___ */