Bug 1486204 - Make nsLookAndFeel.mm return transparent selection colors. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D8210

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

layout/generic/nsTextFrame.cpp

@@ -435,6 +435,10 @@ class nsTextPaintStyle {
   nsSelectionStyle* GetSelectionStyle(int32_t aIndex);
   void InitSelectionStyle(int32_t aIndex);
 
+  // Ensures sufficient contrast between the frame background color and the
+  // selection background color, and swaps the selection text and background
+  // colors accordingly.
+  // Only used on platforms where mSelectionTextColor != NS_DONT_CHANGE_COLOR
   bool EnsureSufficientContrast(nscolor* aForeColor, nscolor* aBackColor);
 
   nscolor GetResolvedForeColor(nscolor aColor, nscolor aDefaultForeColor,
@@ -4018,7 +4022,8 @@ bool nsTextPaintStyle::InitSelectionColorsAndShadow() {
       LookAndFeel::GetColor(LookAndFeel::eColorID_TextSelectForeground);
 
   if (mResolveColors) {
-    // On MacOS X, we don't exchange text color and BG color.
+    // On MacOS X, only the background color gets set,
+    // the text color remains intact.
     if (mSelectionTextColor == NS_DONT_CHANGE_COLOR) {
       nscolor frameColor =
           nsSVGUtils::IsInSVGTextSubtree(mFrame)

widget/cocoa/moz.build

@@ -84,6 +84,7 @@ include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'
 LOCAL_INCLUDES += [
+    '/layout/base',
     '/layout/forms',
     '/layout/generic',
     '/layout/style',

widget/cocoa/nsLookAndFeel.h

@@ -37,6 +37,7 @@ class nsLookAndFeel final : public nsXPLookAndFeel {
   static bool AllowOverlayScrollbarsOverlap();
 
   static bool SystemWantsDarkTheme();
+  static nscolor ProcessSelectionBackground(nscolor aColor);
 
  private:
   int32_t mUseOverlayScrollbars;

widget/cocoa/nsLookAndFeel.mm

@@ -13,6 +13,7 @@
 #include "gfxFont.h"
 #include "gfxFontConstants.h"
 #include "gfxPlatformMac.h"
+#include "nsCSSColorUtils.h"
 #include "mozilla/FontPropertyTypes.h"
 #include "mozilla/gfx/2D.h"
 #include "mozilla/widget/WidgetMessageUtils.h"
@@ -132,6 +133,32 @@ nsLookAndFeel::RefreshImpl()
   mInitialized = false;
 }
 
+// Turns an opaque selection color into a partially transparent selection color,
+// which usually leads to better contrast with the text color and which should
+// look more visually appealing in most contexts.
+// The idea is that the text and its regular, non-selected background are
+// usually chosen in such a way that they contrast well. Making the selection
+// color partially transparent causes the selection color to mix with the text's
+// regular background, so the end result will often have better contrast with
+// the text than an arbitrary opaque selection color.
+// The motivating example for this is the URL bar text field in the dark theme:
+// White text on a light blue selection color has very bad contrast, whereas
+// white text on dark blue (which what you get if you mix partially-transparent
+// light blue with the black textbox background) has much better contrast.
+nscolor
+nsLookAndFeel::ProcessSelectionBackground(nscolor aColor)
+{
+  uint16_t hue, sat, value;
+  uint8_t alpha;
+  nscolor resultColor = aColor;
+  NS_RGB2HSV(resultColor, hue, sat, value, alpha);
+  int factor = 2;
+  alpha = alpha / factor;
+  sat = sat * factor;
+  NS_HSV2RGB(resultColor, hue, sat, value, alpha);
+  return resultColor;
+}
+
 nsresult
 nsLookAndFeel::NativeGetColor(ColorID aID, nscolor &aColor)
 {
@@ -171,12 +198,12 @@ nsLookAndFeel::NativeGetColor(ColorID aID, nscolor &aColor)
       aColor = NS_RGB(0x00,0x00,0x00);
       break;
     case eColorID_TextSelectBackground:
-      aColor = mColorTextSelectBackground;
+      aColor = ProcessSelectionBackground(mColorTextSelectBackground);
       break;
     // This is used to gray out the selection when it's not focused. Used with
     // nsISelectionController::SELECTION_DISABLED.
     case eColorID_TextSelectBackgroundDisabled:
-      aColor = mColorTextSelectBackgroundDisabled;
+      aColor = ProcessSelectionBackground(mColorTextSelectBackgroundDisabled);
       break;
     case eColorID_highlight: // CSS2 color
       aColor = mColorHighlight;