Check out this thread on the DC forum regarding how to turn a plain text file into an outline using Notepad2-mod.
http://www.donationcoder.com/forum/index.php?topic=36417.0

Download Notepad2-mod from here:
http://xhmikosr.github.io/notepad2-mod/

And tweak the settings like this:
http://www.donationcoder.com/forum/index.php?topic=36417.msg340840#msg340840

And you will get an outliner like this one:
http://www.donationcoder.com/forum/index.php?topic=36417.msg340816#msg340816

Could this be the simplest outliner out there yet?

Nice; if you like it, you might also want to check out Emeditor with the outliner plug-in.

installed and modified the settings, and now I see my test text file with proper indentations, and +/- symbols for "parent" lines. I can expand and collapse by clicking on the +/- signs.

does notepad mod provide ways to move lines up/down, promote/demote? keyboard shortcuts for expand/collapse?

Alexander Deliyannis wrote:

Nice; if you like it, you might also want to check out Emeditor with the
outliner plug-in.

Thanks. EmEditor's outliner does look a lot more sophisticated (though it does come with a price tag).

jimspoon wrote:

does notepad mod provide ways to move lines up/down, promote/demote?
keyboard shortcuts for expand/collapse?

Here are some keyboard shortcuts:
https://github.com/XhmikosR/notepad2-mod/wiki/Code-Folding-Usage

looks like notepad2 provides shortcuts for expand/collapse and navigation, but nothing for move up/down/left/right?

jimspoon wrote:

looks like notepad2 provides shortcuts for expand/collapse and
navigation, but nothing for move up/down/left/right?

I don't know. Maybe some kind of a script can do it.

But following Eduardo's tip in the other thread I imported my Notepad2-mod plain-text tab-indented outline into CT's outliner, which does have shortcuts for all that.

jimspoon wrote:

looks like notepad2 provides shortcuts for expand/collapse and
navigation, but nothing for move up/down/left/right?

Actually there are: CTRL+SHIFT+UP and DOWN, and TAB and SHIFT+TAB.

btw, I did come across a little hickup with Notepad2-mod outlining. If you make too many changes in a complex outline while some of the nodes are folded, then some lines may disappear from view even when all get unfolded again. To get the hidden lines back, I switched from the YAML syntax scheme back to Default Text, which fixed it. Then you can return to YAML and switch folding back on, and all is visible again.