transition-group with duration property doesn't work

[yukukotani]

Version

2.6.10

Reproduction link

https://codesandbox.io/s/vue-template-lcrsy

Steps to reproduce

Click "Move" button.

What is expected?

Both two lists move with animation.

What is actually happening?

Only second list which is applied duration with css has animation.

[posva]

duration only exists in transition. Since transition-group has 3 types of animations, we could have something like:

Number: Applies the same duration to enter, leave and move

<transition-group :duration="500">...</transition-group>

Object: Explicit for every type of animation but requires the three of them

<transition-group :duration="{ enter: 500, leave: 800, move: 300 }">...</transition-group>

[yukukotani]

@posva I tried to resolve this: #10627
Please merge if it looks good.