[pvonk]

I'm not sure it makes sense to display "subprojects" in the *left* panel. As dhm points out, they are action groups which behave more like actions that projects. Consider this... action groups may/usually appear with other actions within a list under a project. Like:

Proj
..action
..action
..action group
....action
....action
..action
..action group
....action
....action

Now, what should be displayed in the left panel? ....

Proj
..action group
..action group

Since actions don't show in the left panel, we are left with the project and "subprojects." However, the placement of the action groups may be dependent on the actions around them (esp. for sequential projects). The second list above may be confusing to some under certain circumstances.

Another problem is the nested naturre of action groups. If you get enough of them nested within nested groups, the left panel would get unmanageably wide.