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

Source/core/rendering/style/RenderStyle.cpp

Index: Source/core/rendering/style/RenderStyle.cpp
diff --git a/Source/core/rendering/style/RenderStyle.cpp b/Source/core/rendering/style/RenderStyle.cpp
index e89db8af349ca5009b86f4ad2d9cc54e510963d5..9e576f2ed3487227fe9279c7ec20071dce5b0015 100644
--- a/Source/core/rendering/style/RenderStyle.cpp
+++ b/Source/core/rendering/style/RenderStyle.cpp
@@ -355,6 +355,16 @@ bool RenderStyle::inheritedDataShared(const RenderStyle* other) const
         && 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
@@ -383,7 +393,10 @@ StyleDifference RenderStyle::visualInvalidationDiff(const RenderStyle& other) co
 
     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))