What problem does this feature solve?

When a child leaves, the children after that does not animate. Reproduction link

It does not animate even if leave transitions are not set. Reproduction link

Adding this feature will enable move transition when children are leaving.

What does the proposed API look like?

Add prefer-move-than-leave prop which takes true or false. The default is false. When it is false, it will behave as now. When it is true, it will not do leave transitions but do move transitions.

Implemention

Add if (el._leaveCb) el._leaveCb() here. https://github.com/vuejs/vue-next/blob/03ae3006e1e678ade4377cd10d206e8f7b4ad0cb/packages/runtime-dom/src/components/TransitionGroup.ts#L149-L157

Seems like it is possible by using position: absolute.

link