Toggling binded class causes classes that were added on a custom directive to get removed

[hafezdivandari]

Version

2.6.11

Reproduction link

https://jsfiddle.net/hafez/nobjg618/

Steps to reproduce

1. define a custom directive that adds a class to element's class list.
2. use this defined directive on an element and toggle a random class via v-bind:class

What is expected?

Static classes of the element and classes that are added on directive should stay untouched.

What is actually happening?

When toggling a dynamic class, all classes added on directive get removed.

[posva]

This is expected behavior: Vue cannot detect the dynamically added classes so you should either add all the classes manually in JS or use Vue to handle it.

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[hafezdivandari]

@posva is it documented anywhere?