Ok, created a small project and found out some more info.
The problem seems to be the activity that is the first on the activity stack. This one is never brought to front again.
To reproduce it:
- Install the app.
- exit app with back key (so no activity is on the stack)
- add the widget AATestWidget to the homescreen
- Press button Act2 to start the Activity 2 (green)
- Press Home to return to Homescreen
- Press button Act3 to show Activity 3 (red)
- Press home
- Press button Act2
Activity 3 is then shown.
If you always use the home button to return to the homescreen you can switch between Main and Activity3 from the widget, but Activity2 does not show.
If you start with another activity so this one is the first on the stack then you can't return to this one with the widget.
Perhaps this is a Android problem? I currently Use Gingerbread 2.3.3