Reduction of https://github.com/EmmittJ/PoESkillTree for a bug case (original: PoESkillTree/PoESkillTree#376).
Bug demonstration: http://i.imgur.com/KY0CJrT.gifv (Only reproducible by few users, myself among them, more details in MahApps issue 2809)
I cut out as much as I could get away with. I'm not into the details of WPF/MahApps/Metro so please excuse any obliviousness.