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The group box theme has this bug:
http://stackoverflow.com/questions/151979/does-the-groupbox-header-in-wpf-swallow-mouse-clicks/
I could fix it quite easily by copying your GroupBox.xaml and add IsHitTestVisible="False" to two borders. See http://stackoverflow.com/a/4014508/972084. Perhaps you could fix this bug in your product. Thanks.