I'd like to be able to
:Sex when editing a file, select a file, press CR, and have that new split populated with the file I just selected. Is there an option for this? I think I became used to this behavior when using NERDtree or another plugin.
Well, I have
let g:netrw_browse_split = 4 in my vimrc under the comment "These settings cause Netrw file explorer to behave more like NERDTree" yet I see the same behavior as you're seeing now. So 1) you must be thinking about that other plugin and, more importantly, 2) I'd guess you (or something) is setting that global var somewhere.
What you want is to unset it or set it explicitly to 0.
*g:netrw_browse_split* when browsing, <cr> will open the file by: =0: re-using the same window (default) =1: horizontally splitting the window first =2: vertically splitting the window first =3: open file in new tab =4: act like "P" (ie. open previous window)
I tested with 0 and it works the way you want.