We followed some of the conventions used in Windows Explorer when authoring the new Open toolwindow, but we did make some decisions that departed from their norm. We wanted to make the tree fast for keyboard navigation, and listing the contents of each directory as you move around really slows this down. Once you're highlighting a directory you want, you can press Enter (instead of double-clicking) to make that the active directory and display the file list for it.
We designed it primarily for opening files, with a strong bias toward making it responsive when you're in a directory with a lot of files, or with an open project/workspace with a lot of files.
So even though there is not currently an option to mimic Explorer behavior, we can most likely add one. I'll log it as a feature request.