This is a great plugin, but we have a small conflict on the widgets dashboard. wpmu_dev_maps_plugin/inc/external/wpmu-lib/js/select2.min.js and wpmu_dev_maps_plugin/inc/external/wpmu-lib/js/wpmu-ui.min.js appear to be working together to break multi-select boxes on other widgets. It looks like the MultiSelect2 class is targeting all <select> elements with the multiple property set. I have attached two screenshots, one with Google Maps activated and one with Google Maps deactivated. I marked the differences with blue arrows. I get the same results on the 2015 theme. If you need to test, the Jquery Categories List Widget is free.
I also put together the simplest test of this I could
<select name="select" multiple="multiple">
<option value="value1">Value 1</option>
<option value="value2">Value 2</option>
<option value="value3">Value 3</option>
If you load that as just an html doc in your browser with either select2.min.js or wpmu-ui.min.js alone, the multi-select box is fine. Loaded together though and the following gets injected above the multi-select box
<ul class="select2-choices"> <li class="select2-search-field"> <label for="s2id_autogen2" class="select2-offscreen"></label> <input type="text" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" class="select2-input" id="s2id_autogen2" placeholder="" style="width: 10px;">
Which, when combined with the style sheets (and js, I imagine), causes the behavior in the screenshots.
Thanks for the time.