"Fixed" in the next release. This required changes to multiple templates and files, so patch instructions cannot be provided.
When creating a new category listing in this context, the expected parent category will be automatically selected in the categories field. However, it will be treated as a read-only selection, since you can't remove the context you are currently inside.
This fix also resolves a bug where the "X" was provided in the editor field for categories the user did not have permission to remove entries from. That bug could not have resulted in a permissions escalation because the save action still checked that permission anyway, but now the editor tool doesn't offer to let the user do something that will result in the "no permission" error.