This request is being tracked already:
Issue 612 – Optional expanding of directory tree
You can vote for it there.

Yes, when I enter a directory that contains subfolders, the directory node in the tree is auto-expanded. I'd like to be able to opt out of this behaviour (entering a directory would load its contents in the right pane of WinSCP, and if the directory contains subfolders, its expansion-arrow would appear, but the treenode would not expand automatically).

You mean, when entering the directory?

It would be nice if the automatic expansion of directory tree nodes could be disabled (only show the expansion triangle if the node is expandable)