the problem is very likely in _on_SelectedOneOffButtonChanged()... the down may be actually selecting the one-off for a brief moment, and _on_SelectedOneOffButtonChanged makes assumptions about items and results, in particular it assumes there's a 1:1 relation between them, but that's not true.
Bug 1633030 Comment 8 Edit History
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the problem is very likely in _on_SelectedOneOffButtonChanged()... the down may be actually selecting the one-off for a brief moment, and _on_SelectedOneOffButtonChanged makes assumptions about items and results, in particular it assumes there's a 1:1 relationship between them, but that's not true.