Looks like we can't reproduce the issue or there's a problem in the test case provided.
Ext.tree.Panel bug on column definition with 'hideable: false'
When setting a column to 'hideable: false' with Ext.tree.Panel, the column don´t show on the column list (expected, works like Ext.grid.Panel till here) and the next one is disabled (greyed out - I think this is the old behaviour on 3.x if I recall it right), rendering 2 columns 'unhideable'.
I´m using ext-4.0.7-gpl, and this bug don´t seems related to browsers.
In Ext JS 3.4.0 if you set hideable to false on a column that column won't be shown in the menu. The behavior in Ext JS 4 matches this behavior so the menu item should not show.