On click on any select using Chrome under MacOS there is a bug that doesn't allow a user to select any option. Dropdown is closed instead immediately.
As a solution you may change
Event.on(this.selectElement, 'click', function (event) { self.onTrigger(event); });
in file rtehtmlarea/Resources/Public/JavaScript/HTMLArea/Toolbar/Select.js:103
to
Event.on(this.selectElement, 'change', function (event) { self.onTrigger(event); });