I would like a (switchable) explorer behavior: when 
opening a folder, in tree-view, it would recursively open 
any/all sub-folders if total child nodes is less/equal 
than some threshold, typically 1. 

The heuristic is that the user is typically interested in 
the "leaves" of the tree, if the branches are empty.

This would optimize getting down to source code in a deep 
package layout. eg:
- com
  - mydomain
    - my project
       - my package
         - My file 1
         - My File 2

would open automatically to "mypackage", rather than 
to "com", because it is the first sub-node with multiple 
children. 

While not a huge change, explorer navigation is so 
universal and common that speeding this process would be 
very helpful for most user's productivity.

Theres no reason why the threshold need be 1. Some users 
may prefer eg. 2 or 5 children as the limit of recursive 
opening. Better as a settable param with default=1.