In acme it is easy to visually grok where to click if you want to resize a win/col or move a win to another col.

By comparison, there is no quick visual way of determining where to click to resize columns or windows or for moving windows to another column, see image below from a.

I don't really have any opinion as to how to indicate these two areas, just that they are easy to distinguish from each other and from the scroll bar when the window above has a scroll handle that isn't the size of the window itself, see image below.

Also, thanks for the excellent work on this project 👍