Since upgrading to 2.5 and finding the write page layout highly inefficient, I moved the categories into the sidebar next to the edit pane, as they were in 2.3.3. You can download my revised edit-form-advanced file here.
Since I moved it to the sidebar, the ability to add a new category (or type in an existing category) doesn't work, returning an error that I don't have permission to do that. If I restore the original edit-form-advanced file, I do indeed have permission. What is missing in moving the add-category form to the sidebar?