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

Source/core/editing/FrameSelection.cpp

Index: Source/core/editing/FrameSelection.cpp
diff --git a/Source/core/editing/FrameSelection.cpp b/Source/core/editing/FrameSelection.cpp
index aa4a95d3d3c4f540af58cdf7384ca816c0017406..2016ea8755934df0daecf7b6380321bbcbbef607 100644
--- a/Source/core/editing/FrameSelection.cpp
+++ b/Source/core/editing/FrameSelection.cpp
@@ -1440,9 +1440,29 @@ void CaretBase::paintCaret(Node* node, GraphicsContext* context, const LayoutPoi
 
     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();
     }