Fix overconstrained behaviour for position: sticky.

Source/core/rendering/RenderBoxModelObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 301 matching lines @@
 
     if (containingBlock->hasOverflowClip()) {
         IntSize scrollOffset = containingBlock->layer()->scrollableArea()->adjustedScrollOffset();
         stickyLocation -= scrollOffset;
     }
 
     // We can't call localToAbsolute on |this| because that will recur. FIXME: For now, assume that |this| is not transformed.
     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);
     }
 }
 
 LayoutSize RenderBoxModelObject::stickyPositionOffset() const
 {
     FloatRect constrainingRect;
 
     ASSERT(hasLayer());
     RenderLayer* enclosingClippingLayer = layer()->enclosingOverflowClipLayer(ExcludeSelf);
@@ skipping 2475 matching lines @@
     ASSERT(!beforeChild || toBoxModelObject == beforeChild->parent());
     for (RenderObject* child = startChild; child && child != endChild; ) {
         // Save our next sibling as moveChildTo will clear it.
         RenderObject* nextSibling = child->nextSibling();
         moveChildTo(toBoxModelObject, child, beforeChild, fullRemoveInsert);
         child = nextSibling;
     }
 }
 
 } // namespace WebCore