I have the same problem. I proposed more compact solutions, including:

1. Small vertical spacing between groups just to break things up
1b. possibly with a thin 1px line in that space 

(for instance, these would be underneath 'favorites' and before 'slack' in your image

2. thin lines to the left, with breaks in the same places as the spaces in (1), in differing colors--the color coding not meaning anything particular but just giving you more information about groups that are going on. 
3. Normal treeview lines, more or less exactly where you put your gaps, except that the gaps would only be large enough for the lines.