Quote: However (and this is probably a very common case), if I have Content-focus and the editor is focussing on a note, the user who would want to click another note and not know your proposed behaviour could be frustrated, since in that case down/up-arrow would already have it's normal effect. You could make it uniform, so that an editible attachment never has text-cursor focus, but that would probably annoy people, who don't want to navigate up/down, but just want to edit their notes fast.They'd all be forced to do one extra mouse click. The current behavior is inconsistent: If you have a non-text attachment selected, then you click on a text attachment, you have to click inside the content area to focus it for editing. If you have a text-attachment selected and your content area is focused, clicking a different text attachment will immediately focus the content area for editing. If you have a text-attachment selected and the the content area is not focused, clicking a different text attachment will not focus the content area.
In my view, it would be best to keep the attachment list completely separate from the content area, so yes the user would have to click in the content area to start editing. I don't think that's a terrible inconvenience and from the UI perspective, it provides an intuitive and consistent result. When an attachment is focused, you can perform operations on it, including navigating to other attachments using the arrow keys. And when the content area is focused, any operations would apply to the content area.