Testing on party category tree view:

• I start with no categories. I click on “New” and create category “a”.
• Press “New” again and create category “b”. I cannot add it as a child of a (parent is readonly).
• I move to category “a” and create two children “aa” and “ab”.
• I switch to tree view.
• Now if I press “New” when the “a” category is selected, the parent field of the new record is readonly (record must be sibling of a) but if I press “New” when the “aa” category is selected, the parent field of the new record is readwrite.

I guess the expected behaviour in the last case is that the parent field should be readonly.

Following: https://discuss.tryton.org/t/menu-tree-vs-list-view/