Is there any way to set the size (width and/or height) of the pane border separating the folder tree and panes?
I would really like it to be a single pixel wide, but I can also imagine situations in which it'd be nice to have it be bigger so that there is more separation between the sections instead.
Not currently possible. The visual width matches the area you can grab with the mouse (although it doesn't technically have to do that, of course... but the way the panel borders interact with each other is already complex enough!)
It's not just a pixel value that determines the size, which could possibly be increased or decreased?
In case you meant that the grab area itself is a fixed size, that would only make it act a little weird if the border size would be increased (like this), that only a portion in the middle of the bar would be grabbable and draggable, which may still be a fair and understandable compromise. The current grab area would still work well with a smaller pane border size, with the cursor overlapping plenty on either side.
Unless the size is heavily tied into other things of course.
I've changed this into a feature request. Thanks for the response and consideration regardless!
It's much more complex than just adjusting a size, since making the hitbox 1 pixel would make it difficult to grab the slider (or even impossible, depending on mouse settings and DPI).
To make it look 1 pixel wide while keeping the original hitbox, we'd have to make the rest of the splitter render itself so it looks like the folder tree or file display, which could have anything in them (including background images). Not impossible, but it would mean a lot of added complexity.