When opening a single file for a quick edit, it should be clear the window holds a file and not a project. On the other hand, when opening a folder , the sidebar should be open and show the contents of that folder.
This principle aplies and can be currently observed in TextMate:
The available "Toggle Side Bar" should only affect the frontmost window and not change the behavior of windows opened after that.
The default state of the sidebar in each case (file/folder), could be configurable in the sublime-settings preferences in order to accomodate specific workflows.
(this post might seem duplicate but it's an attempt of clarification of a previous feedback which was misunderstood)
Customer support service by UserEcho