Extracted from bug 178865. Almost all layout functions in MathML ignore out-of-flow children, except RenderMathMLRow. Consider for example <math> <mtext style="position: absolute; left: 100px; top: 100px;">A</mtext> <mfrac> <mtext style="position: absolute; left: 100px; top: 100px;">B</mtext> <mtext></mtext> </mfrac> </math> where A is absolutely positioned but the numerator B is not. It's not clear whether it's really an important use cases. Maybe we should add special insertPositionedObject calls in all MathML layout functions to be compatible with the CSS behavior. Or maybe we should remove the special handling in RenderMathMLRow to reduce code complexity.