FileSystem dock will switch to tree mode when opening any directory

[kubecz3k]

Godot version:
b068961
OS/device including version:
Ubuntu 17.04
Issue description:
When using FileSystem dock without tree view it's impossible to open any directory. Instead tree view will turn on and directory which should be open will be selected:

I'm quite sure it's a recent regression (I think it can't be older than 10 days)

Steps to reproduce:

1. Move filesystem dock below the scene tree dock
2. Set its size so the tree view will be invisible
3. Try to open any directory

Minimal reproduction project: