While utilizing the Layout Mix internally here at IdentityMine we found a small issue which we thought was worth fixing. A description of the change from one of our engineers:
MakeVisible method in StackPanel and StackedStackPanel may not work correctly with some layouts. Now the function is more reliable now that is uses the contstraint size of the panel rather than its desired or rendered size.
Because it is small issue we have left the version at 1.0 and thus there is not need to upgrade your product key to get the fix. You can find the updated build here: http://blendables.com/files/folders/layout_installer/entry595.aspx
Thanks all,
Kurt