I can't reproduce this in MindManager 2017 - when sliding the split it won't let me reduce the Notes window below a certain width (about 3 cm).
In any case, what's wrong with clicking on the Close button in the top right corner of the Notes window to close it, or toggling Control-T to close or open the Notes window?
This is without any other Taskpane open including the Notes.
Hope this helps.
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