Change the caret color according to the lightness of the background color.

Source/core/editing/FrameSelection.cpp

 /*
  * Copyright (C) 2004, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
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  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1422 matching lines @@
     RenderObject* renderer = caretRenderer(node);
     if (renderer && renderer->isBox())
         toRenderBox(renderer)->flipForWritingMode(drawingRect);
     drawingRect.moveBy(roundedIntPoint(paintOffset));
     LayoutRect caret = intersection(drawingRect, clipRect);
     if (caret.isEmpty())
         return;
 
     Color caretColor = Color::black;
     ColorSpace colorSpace = ColorSpaceDeviceRGB;
-    Element* element = node->rootEditableElement();
+    Element* element = static_cast<Element *>(node);
     if (element && element->renderer()) {
-        caretColor = element->renderer()->style()->visitedDependentColor(CSSPropertyColor);
+        bool isBackgroundWhite = true;

[eseidel, 2013/04/27 05:10:42]

This doesn't mean "isBackgroundWhite" either. :)  I guess your previous usage
was more correct, now that I'm understanding what this does.

+        RenderObject* renderer = element->renderer();
+        while (renderer) {

[eseidel, 2013/04/27 05:10:42]

I would re-write this as a helper function:

RenderObject* closestAncestorWithBackground(RenderObject*);

Then this becomes:

RenderObject* opaqueAncestor =
closestAncestorWithBackground(element->renderer());

That said, I dont' think this code will actually work in all cases.  Take for
example an absolutely positioned div.  It may be positioned outside of it's
container and may end up with the "wrong" caret color.

I think to do this correctly, you'd have to actually hit-test the caret
location.  But I'm not sure this is really worth the complexity.

Do we have compatibility concerns driving this?  Sites which depend on this
behavior?  Or is this just a UI preference?

[joone, 2013/04/29 19:17:28]

Okay, I will add this helper function. 
I didn't think of this case. Can we get the actual background color from Skia? I
think we can change the caret color according to the screen color if this is
easier than hit-testing the caret color.
There seems no compatibility issues because each browser engine works
differently. I think this is an accessibility issue or usability issue. I heard
that black/white caret is more visible to people who have color blindness 

+            if (renderer->style()->hasBackground()) {
+                isBackgroundWhite = false;
+                break;
+            }
+            if (!renderer->isRoot())
+                renderer = renderer->parent();
+            else
+                break;
+        }
+
+        Color backgroundColor;
+        if (!isBackgroundWhite) {
+            backgroundColor = renderer->style()->visitedDependentColor(CSSPropertyBackgroundColor);
+            if (backgroundColor.lightness() < 0.5)
+                caretColor = Color::white;
+        } else
+            caretColor = Color::black;
+
         colorSpace = element->renderer()->style()->colorSpace();
     }
 
     context->fillRect(caret, caretColor, colorSpace);
 }
 
 void FrameSelection::debugRenderer(RenderObject *r, bool selected) const
 {
     if (r->node()->isElementNode()) {
         Element* element = static_cast<Element *>(r->node());
@@ skipping 593 matching lines @@
     sel.showTreeForThis();
 }
 
 void showTree(const WebCore::FrameSelection* sel)
 {
     if (sel)
         sel->showTreeForThis();
 }
 
 #endif