Why is the back navigation function of the navigation controller not working when both fragments are displayed simultaneously in the activity?

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The back navigation function of the navigation controller may not work when both fragments are displayed simultaneously in the activity because the navigation controller maintains a stack of destinations that allows it to navigate back to the previous destination using the back button. However, when two fragments are displayed simultaneously, it is unclear which fragment should be removed from the stack when the back button is pressed, leading to unexpected behavior. To fix this issue, one possible solution is to use a different layout, such as a ViewPager or a FrameLayout, to display the fragments one at a time, ensuring that the navigation controller can properly manage the back stack.