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Activity Y has 'singleTask' launch mode but it is not root activity in the task, that is Activity X. How did it happen?
Upd.:
From the documentation:
"singleTask"
The system creates a new task and instantiates the activity at the root of the new task. However, if an instance of the activity already exists in a separate task, the system routes the intent to the existing instance through a call to its onNewIntent() method, rather than creating a new instance. Only one instance of the activity can exist at a time. Note: Although the activity starts in a new task, the Back button still returns the user to the previous activity.