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 05743f6f538712356fb96c67ee003de927682691..231fde0c22179ed525fd0be2416e7f6cd9ba7c06 100644
--- a/Source/core/rendering/RenderBoxModelObject.cpp
+++ b/Source/core/rendering/RenderBoxModelObject.cpp
@@ -329,15 +329,34 @@ void RenderBoxModelObject::computeStickyPositionConstraints(StickyPositionViewpo
         constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeRight);
     }
 
+    float offsetsSum = constraints.rightOffset() + constraints.leftOffset();

[Julien - ping for review, 2014/05/16 13:17:49]

We should rename this to a better name: verticalOffsets

[ostap, 2014/05/22 04:33:19]

Done.

+    if (offsetsSum > containerContentRect.width().toFloat()
+        || offsetsSum + containerContentRect.width().toFloat() > constrainingRect.width()) {
+        if (style()->isLeftToRightDirection()) {
+            constraints.setRightOffset(0);
+            constraints.removeAnchorEdge(ViewportConstraints::AnchorEdgeRight);
+        } else {
+            constraints.setLeftOffset(0);
+            constraints.removeAnchorEdge(ViewportConstraints::AnchorEdgeLeft);

[Julien - ping for review, 2014/05/16 13:17:49]

It would be cleaner if we didn't set the anchor edge in the first place.

The logic doesn't work if we set the right offset with a LTR direction so that
offsetsSum is bigger that the containing block's width. This is an undefined
behavior in the specification but I would have liked to see this mentioned.

[ostap, 2014/05/22 04:33:19]

Done
Actually, the over-constrained means that both left and right or top and bottom
edges are not auto (similar to relative positioning). This check was actually
missed from my 1st patch and I fixed this.
When only one edge offset is bigger than containing block is a different case
and existing calculations do it properly similar to mozilla implementation and
as it was discussed on the CSS mailing list here:
http://lists.w3.org/Archives/Public/www-style/2014Mar/0141.html
It seems this case (with single offset bigger than container) is not covered by
any existing layout test, but it is not related to this patch and I would make
it separately.

+        }
+    }
+
     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() ));
+        constraints.setBottomOffset(floatValueForLength(style()->bottom(), constrainingRect.height()));
         constraints.addAnchorEdge(ViewportConstraints::AnchorEdgeBottom);
     }
+
+    offsetsSum = constraints.topOffset() + constraints.bottomOffset();
+    if (offsetsSum > containerContentRect.height().toFloat()
+        || offsetsSum + containerContentRect.height().toFloat() > constrainingRect.height()) {
+        constraints.setBottomOffset(0);
+        constraints.removeAnchorEdge(ViewportConstraints::AnchorEdgeBottom);
+    }

[Julien - ping for review, 2014/05/16 13:17:49]

Shouldn't you check if our writing mode is not bottom-to-top? This is similar to
what you did above for RTL direction.

[ostap, 2014/05/22 04:33:19]

The over-constrained behavior is done similar to position:relative. If we change
it here then, I think, relative positioning should be also changed and spec.
updated. But I'll ask on mozilla bug report which has related discussion:
https://bugzilla.mozilla.org/show_bug.cgi?id=979602

 }
 
 LayoutSize RenderBoxModelObject::stickyPositionOffset() const