Mariner Software StoryMill for Mac - Manual (Page 40)

Splitting windows
If you are working in a content pane, you can split your text view to
both edit and reference the item at the same time.
With an editing window open, choose View→Split
Horizontally or View→Split Vertically.
Drag the bar between the splits to resize them to your liking.
Choose View→Unsplit to remove the split.
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Tip:
You can also use the button that
sits on top of the scrollbar to split
your text view!