UPDATE June 2018
You can avoid the bug described in this article by using Safari 11.1 on MacOS 10.13. Because of this, specifying Safari '11.0' AND 'MacOS 10.13' as desired capabilities is no longer an available combination. Specifying Safari '11', '11.1', or not specifying a Safari version along with the MacOS 10.13 platform, will now default to Safari '11.1'.
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When testing with Safari 11 and Selenium 3.x, you may find that click events on option elements (such as dropdown menus) are not handled correctly, making it appear as if the click does not occur or register correctly.
This is a WebKit bug, not a Sauce Labs bug. You can find details at https://bugs.webkit.org/show_bug.cgi?id=174710. This bug has been fixed, but Apple have yet to fix this in SafariDriver. This has been reported to Apple under bug report number 33459305.