So, after playing around and a another few looks into the qt touch-gallery example, where they do exactly the with on of their buttons, I found a working solution and maybe a bug in the StackView.
But thats I'm not really sure of. Maybe someone with more experience in QtQuick and StackView can confirm it ;)
After adding a Text in App1 that displays the current stackdepth, I was surprised it shows a value of 2 even when I've only added one Item.
And from this visibility of the depth, I was able to see that the call if (appStack) appStack.pop() call at onReleased: in App1 is actually working. The last Item was popped so that that there was only one left on the stack. As those were both yellow I havn't seen the pop effect before.
What I'm not sure about now:
As I havn't found a rule, that a StackViewalways needs a initialItem, is it working as intended, that the first item to be added on a empty StackView is added twice?
Is it working as intended, that the last item on the Stackview can't be popped with pop() but must be popped with clear() ?
If anyone with more experience in QtQuick and the usage of StackView knows anything about this points, feel free to explain your knowledge/thougths.
If this is really a bug in StackView I would report it after confirmation ;)
PS: as a workaround for now I'm using onReleased: appStack.depth > 1 ? appStack.pop() : appStack.clear() in App1.
Like this both instances on the Stack are popped at the same time.