What I wanted to achieve was this:

    transitions: [
        Transition {
            from: "hidden"; to: "visible";
            SequentialAnimation {
                PauseAnimation { duration: 250 } // time for the other page to hide
                PropertyAnimation { duration: 250; property: "opacity"; }
            }
        },
        Transition {
            from: "visible"; to: "hidden";
            PropertyAnimation { duration: 250; property: "opacity"; }
        }
    ] 

However, when I used Transitions tab to achieve this, the code ended up generated looked like this: (Somehow, the code copied from QDS breaks its format when pasted here, hense the screenshot)

And currently there's no other way to remove them but to do so in the Code editor.

For ParallelAnimation, if there's only one Animation, in this case SequentialAnimation, it should be automatically removed.
For PauseAnimation, if the duration is 0, it should be automatically removed.