What is the dividing line between the directory tree and the right-side view called? Specifically, what is the name of the "grab it and slide/resize" function? I'm trying to make this in wxPython but I don't know the name of it.
If you're looking for an interaction pattern name:
It's just a column or pane divider. Whether or not it's resizable is up to you.
The general arrangement is a sort of modified Miller Column or master detail UI.
If you're looking for the code function to call:
You came to the wrong SE site.