Fix overconstrained behaviour for position: sticky.

Source/core/rendering/RenderBoxModelObject.cpp

Index: Source/core/rendering/RenderBoxModelObject.cpp
diff --git a/Source/core/rendering/RenderBoxModelObject.cpp b/Source/core/rendering/RenderBoxModelObject.cpp
index f4dacfa008004a50214b69139cf1b2e848fa3eaf..a71b4f50c21d6aee02d162875b4214a6b61a802e 100644
--- a/Source/core/rendering/RenderBoxModelObject.cpp
+++ b/Source/core/rendering/RenderBoxModelObject.cpp
@@ -319,23 +319,43 @@ void RenderBoxModelObject::computeStickyPositionConstraints(StickyPositionViewpo
     FloatRect absoluteStickyBoxRect(absContainerFrame.location() + stickyLocation, flippedStickyBoxRect.size());
     constraints.setAbsoluteStickyBoxRect(absoluteStickyBoxRect);
 
-    if (!style()->left().isAuto()) {
+    float horizontalOffsets = constraints.rightOffset() + constraints.leftOffset();
+    bool skipRight = false;
+    bool skipLeft = false;
+    if (!style()->left().isAuto() && !style()->right().isAuto()) {
+        if (horizontalOffsets > containerContentRect.width().toFloat()
+            || horizontalOffsets + containerContentRect.width().toFloat() > constrainingRect.width()) {

[Julien - ping for review, 2014/05/22 15:49:02]

[It's better to use a generated 'reply' comment, that avoids having the reviewer
do some copy & paste which loses a lot of the context :( ]
I am fine with that (I usually ask for a FIXME). It seems to me that CSS 2.1 is
clear on what would happen in this case so we may as well just match the in flow
case and have some coverage for it.

+            skipRight = style()->isLeftToRightDirection();
+            skipLeft = !skipRight;
+        }
+    }
+
+    if (!style()->left().isAuto() && !skipLeft) {
         constraints.setLeftOffset(floatValueForLength(style()->left(), constrainingRect.width()));
         constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeLeft);
     }
 
-    if (!style()->right().isAuto()) {
+    if (!style()->right().isAuto() && !skipRight) {
         constraints.setRightOffset(floatValueForLength(style()->right(), constrainingRect.width()));
         constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeRight);
     }
 
+    bool skipBottom = false;

[Julien - ping for review, 2014/05/22 15:49:02]

Let's add a FIXME about that and ideally a link to the www-style change request.

[ostap, 2014/05/22 23:13:11]

Done.

+    float verticalOffsets = constraints.topOffset() + constraints.bottomOffset();
+    if (!style()->top().isAuto() && !style()->bottom().isAuto()) {
+        if (verticalOffsets > containerContentRect.height().toFloat()
+            || verticalOffsets + containerContentRect.height().toFloat() > constrainingRect.height()) {
+            skipBottom = true;
+        }
+    }
+
     if (!style()->top().isAuto()) {
         constraints.setTopOffset(floatValueForLength(style()->top(), constrainingRect.height()));
         constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeTop);
     }
 
-    if (!style()->bottom().isAuto()) {
-        constraints.setBottomOffset(floatValueForLength(style()->bottom(), constrainingRect.height() ));
+    if (!style()->bottom().isAuto() && !skipBottom) {
+        constraints.setBottomOffset(floatValueForLength(style()->bottom(), constrainingRect.height()));
         constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeBottom);
     }
 }