Having done this, you now find yourself looking at a screen with a tree structure of the document on the left, pages of the document on the right, and three tabs at the bottom: Data Output, Data Editor, Title List. The right pane is divided into two. If you create a document with lots of sub headings, and then use the tabs, the power of the application becomes apparent. You have two windows into the document. The top scrolling pane, assuming you have selected a node which has child nodes, will have the beginning text. The bottom scrolling pane will have the subsections one after the other. The main difference between the Output and the Editor mode is that in one you can read, and in the other you can edit, and the Output gives more of the screen over to the text. The Title list is only going to be useful in a document of some complexity, but it gives a complete breakdown of the headings, but in the main panes, not simply at the side.
You need a fairly big screen to make the most of this. But assuming this is available, the ability to see both the main text and the section text in two independent windows, and to resize the relative size of each, makes for a powerful revision tool. For people used to traditional word processors and scrolling linearly through a document, it takes getting used to, but it is worth it.
There are some limitations to this. If you are looking at the whole document tree, you will see the sucessive daughter nodes one level in, in two scrolling windows. But you will not see the text which is in subsections of these daughter nodes. To see that, you have to go to the particular daughter nodes. This happens in both editing and output modes, and is a disadvantage for this purpose. One really wants the option to see the entire document in linear view, just not for that to be the only view. But this is a common enough feature of these kinds of packages. In kjots, for instance, you have the table of contents of a particular document, followed by a scrolling list of its contents, but there is no way to get the entire contents of the tree as a scrolling list.
On balance, Treeline is a real contender. I guess the support requirements would be higher than with Kate, and might amount to several tutorial sessions, with practice in between. Your writer is going to have to be motivated. Stability, as opposed to user interface features, is a critical aspect of using such a package for a 500 page book. It has not crashed and lost data, but I have not done stress testing.
Treeline has lots of features which are not relevant to our purpose, and which make it an outstanding choice for a personal data & clippings organiser. It is more complex than any of the other similar packages, but far more highly featured. For example, the last revision supports conditional fields. However, all that is outside the scope of this note.
Conclusions
If Lyx had two things, it would be the automatic choice. One is rtf export and if possible import. The other is outlining manipulation, the ability to move sections around and have the numbering automagically adjust. Even without this, you don't have to use it very long to understand its popularity. The great advantage is that you can structure your document properly, without having to worry about the aesthetics of any particular implementation of that structure in typesetting. This is an enormous time saver over any traditional word processing package. Used like this, it should be fairly easy to learn - a lot easier than most word processors. Used otherwise, ie with customized classes, the learning curve will be vertical. It is very fast and very stable. A second advantage is that the onscreen layout as you compose is attractive. There is mininal toolbar clutter, and the various headings look right and structure the document on screen in an easy and intelligible way.
It will be important to positively teach the use of Lyx, because although basic use is easy, it is so different from what most people are used to. But as long as you can remain with the predefined formats, it can be taught in a few hours. It is well worth the effort to see if you like it.
