So I was trying to use the “Split at selection” option, and I selected the text I wished to turn into a new text file within the current one.
It keeps grabbing everything below the selected area.
I wish it would still keep the original content on the page I split from because, for example, if you had a detailed description for something and a brief description on the original page, this option would make things easier for some users.
I think you want the Edit → Append Selection to Document command instead. As the name suggests, it lets you create a new document from the selected text but leaves the current document alone.
If you’re trying to excise a section from the middle of a document, the solution is to split twice, before and after the section. Move the chunk you want to excise, then use Document → Merge to glue the remaining two pieces back together.
You can also just drag and drop any selected text out to the binder, corkboard or outliner views to create a new chunk with that text. Though that option does also snip the text out of the original—and the Windows version never had the option added to not do that—so you do have to go and Undo from the original document, if your intention was entirely to copy rather than snip.
Splitting does only exactly what that word means: takes one thing and makes two things out of it. Your choice is only in where to make that break. It is certainly not a bug, as this was originally filed, that it works that way.