Best suitable multiple select with optgroup UI element

Question

My website has a drop-down list that displays all countries, grouped by continent. A user has to select all the countries he needs, as shown in the following figure:

The main issue here is the user cannot click on a continent label and select all its countries. Is there a better solution to this problem?

Answer 1

Continent label has a different style than the rest of the list, it has a header style so it will be strange if it's also clickable.

An easy win could be to include in the first row of each continent group something like: "All countries", in the way when it's clicked will select the whole continent.