Bug 1476524 - Don't require so much contrast for selection background-against-background checks. r=jfkthame

Differential Revision: https://phabricator.services.mozilla.com/D111794

layout/base/nsCSSColorUtils.h

@@ -15,6 +15,12 @@
 // "Techniques For Accessibility Evalution And Repair Tools".
 // See http://www.w3.org/TR/AERT#color-contrast
 #define NS_SUFFICIENT_LUMINOSITY_DIFFERENCE 125000
+// NS_SUFFICIENT_LUMINOSITY_DIFFERENCE is the a11y standard for text
+// on a background. Use 20% of that standard since we have a background
+// on top of another background
+#define NS_SUFFICIENT_LUMINOSITY_DIFFERENCE_BG \
+  (NS_SUFFICIENT_LUMINOSITY_DIFFERENCE / 5)
+
 #define NS_LUMINOSITY_DIFFERENCE(a, b)                    \
   int32_t(mozilla::Abs(NS_GetLuminosity(a | 0xff000000) - \
                        NS_GetLuminosity(b | 0xff000000)))

layout/generic/nsTextFrame.cpp

@@ -3837,10 +3837,18 @@ bool nsTextPaintStyle::EnsureSufficientContrast(nscolor* aForeColor,
   InitCommonColors();
 
   // If the combination of selection background color and frame background color
-  // is sufficient contrast, don't exchange the selection colors.
-  int32_t backLuminosityDifference =
+  // has sufficient contrast, don't exchange the selection colors.
+  //
+  // Note we use a different threshold here: mSufficientContrast is for contrast
+  // between text and background colors, but since we're diffing two
+  // backgrounds, we don't need that much contrast.  We match the heuristic from
+  // NS_SUFFICIENT_LUMINOSITY_DIFFERENCE_BG and use 20% of mSufficientContrast.
+  const int32_t minLuminosityDifferenceForBackground = mSufficientContrast / 5;
+  const int32_t backLuminosityDifference =
       NS_LUMINOSITY_DIFFERENCE(*aBackColor, mFrameBackgroundColor);
-  if (backLuminosityDifference >= mSufficientContrast) return false;
+  if (backLuminosityDifference >= minLuminosityDifferenceForBackground) {
+    return false;
+  }
 
   // Otherwise, we should use the higher-contrast color for the selection
   // background color.
@@ -4023,7 +4031,9 @@ bool nsTextPaintStyle::GetSelectionUnderlineForPaint(int32_t aIndex,
 }
 
 void nsTextPaintStyle::InitCommonColors() {
-  if (mInitCommonColors) return;
+  if (mInitCommonColors) {
+    return;
+  }
 
   nsIFrame* bgFrame = nsCSSRendering::FindNonTransparentBackgroundFrame(mFrame);
   NS_ASSERTION(bgFrame, "Cannot find NonTransparentBackgroundFrame.");

widget/gtk/nsLookAndFeel.cpp

@@ -213,10 +213,7 @@ static bool GetBorderColors(GtkStyleContext* aContext, nscolor* aLightColor,
 // from both Highlight, used as focused+selected background, and the listbox
 // background which is assumed to be similar to -moz-field
 nsresult nsLookAndFeel::InitCellHighlightColors() {
-  // NS_SUFFICIENT_LUMINOSITY_DIFFERENCE is the a11y standard for text
-  // on a background. Use 20% of that standard since we have a background
-  // on top of another background
-  int32_t minLuminosityDifference = NS_SUFFICIENT_LUMINOSITY_DIFFERENCE / 5;
+  int32_t minLuminosityDifference = NS_SUFFICIENT_LUMINOSITY_DIFFERENCE_BG;
   int32_t backLuminosityDifference =
       NS_LUMINOSITY_DIFFERENCE(mMozWindowBackground, mFieldBackground);
   if (backLuminosityDifference >= minLuminosityDifference) {