Use case 1:

• Open a content
• Select a mixin in the gallery : the mixin is affected to content and the mixin is toggle in gallery => OK
• Try to remove the mixin => KO nothing append

Use case 2:

• Open a content with a mixin
• Remove the mixin => OK the mixin is untoggle in gallery
• Try to add the same mixin => KO a confirm "are you sur to remove thsi mixins" is display. We need to refresh browser to re-add the mixin