'Halna Outliner' is a simple outliner app which has two panes.
And you can resize the width (or height) of the pane.
Colors of fonts,backgrounds,etc can be customized.
It has Useful toolbar
You can switch 4 toolbars and make it double decked.
And they are customizable.
With a phone which has a relatively small screen
you can use the app in "switch" style in which you switch panes by tapping the button.