I have test cases that cover the following scenario
I am on the instance page
I select a activity instance in the tree which has siblings
I can see test that cover the following selection behaviour.
1. I click on the flow node ---> all activity instances are selected
2. I click on one of the selected activity instances ---> only this instance is selected
3. I click on the selected instance again ---> it is unselected and the selection jumps back to the default root node).