When a positioned object's dependence on content height changes we need to layout.

Source/core/rendering/style/RenderStyle.cpp

 /*
  * Copyright (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Adobe Systems Incorporated. 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
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 337 matching lines @@
 
 bool RenderStyle::inheritedDataShared(const RenderStyle* other) const
 {
     // This is a fast check that only looks if the data structures are shared.
     return inherited_flags == other->inherited_flags
         && inherited.get() == other->inherited.get()
         && m_svgStyle.get() == other->m_svgStyle.get()
         && rareInheritedData.get() == other->rareInheritedData.get();
 }
 
+static bool dependenceOnContentHeightHasChanged(const LengthBox& a, const LengthBox& b)
+{
+    // If top or bottom become auto/non-auto then it means we either have to solve height based
+    // on the content or stop doing so (http://www.w3.org/TR/CSS2/visudet.html#abs-non-replaced-height)
+    // - either way requires a layout.
+    if (a.top().isAuto() != b.top().isAuto() || a.bottom().isAuto() != b.bottom().isAuto())

[mstensho (USE GERRIT), 2015/02/02 10:00:47]

Just "return blablabla", instead of "if (blablabla) return true; return false"?

But shouldn't you really be using logicalTop() and logicalBottom() here? Adding
a test for vertical writing mode would be nice, in any case.

Also, this may result in false positives (mark for layout when it's not really
needed) in over-constrained situations (when height != auto), but I don't think
we need to worry about that.

[rhogan, 2015/02/02 19:22:49]

We would still need to resolve top/bottom in computeLogicalHeight if they were
becoming auto and the height was non-auto. So maybe not *that* many false
positives. :)

+        return true;
+    return false;
+}
+
 StyleDifference RenderStyle::visualInvalidationDiff(const RenderStyle& other) const
 {
     // Note, we use .get() on each DataRef below because DataRef::operator== will do a deep
     // compare, which is duplicate work when we're going to compare each property inside
     // this function anyway.
 
     StyleDifference diff;
     if (m_svgStyle.get() != other.m_svgStyle.get())
         diff = m_svgStyle->diff(other.m_svgStyle.get());
 
     if ((!diff.needsFullLayout() || !diff.needsPaintInvalidation()) && diffNeedsFullLayoutAndPaintInvalidation(other)) {
         diff.setNeedsFullLayout();
         diff.setNeedsPaintInvalidationObject();
     }
 
     if (!diff.needsFullLayout() && diffNeedsFullLayout(other))
         diff.setNeedsFullLayout();
 
     if (!diff.needsFullLayout() && surround->margin != other.surround->margin) {
         // Relative-positioned elements collapse their margins so need a full layout.
         if (position() == AbsolutePosition || position() == FixedPosition)
             diff.setNeedsPositionedMovementLayout();
         else
             diff.setNeedsFullLayout();
     }
 
     if (!diff.needsFullLayout() && position() != StaticPosition && surround->offset != other.surround->offset) {
         // Optimize for the case where a positioned layer is moving but not changing size.
-        diff.setNeedsPositionedMovementLayout();
+        if (dependenceOnContentHeightHasChanged(surround->offset, other.surround->offset))
+            diff.setNeedsFullLayout();
+        else
+            diff.setNeedsPositionedMovementLayout();
     }
 
     if (diffNeedsPaintInvalidationLayer(other))
         diff.setNeedsPaintInvalidationLayer();
     else if (diffNeedsPaintInvalidationObject(other))
         diff.setNeedsPaintInvalidationObject();
 
     updatePropertySpecificDifferences(other, diff);
 
     // Cursors are not checked, since they will be set appropriately in response to mouse events,
@@ skipping 1260 matching lines @@
         horizontal || includeLogicalRightEdge);
 
     edges[BSLeft] = BorderEdge(borderLeftWidth(),
         visitedDependentColor(CSSPropertyBorderLeftColor),
         borderLeftStyle(),
         borderLeftIsTransparent(),
         !horizontal || includeLogicalLeftEdge);
 }
 
 } // namespace blink