'Vue transition:Why is there no transition animation when switching between nested child routes

When I switching between /test/c and /test/d, about to disappear component not in the DOM Tree, I guess is it something to do with them having a common parent component. I had expected like from /a to /b leave's component have leaving animation, how can I fixed this problem?

Here is demo: codesandbox.io



Solution 1:[1]

You must add a key to the parent <router-view :key="$route.path">

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Source: Stack Overflow

Solution Source
Solution 1 IVO GELOV