Date popup field gets hidden when field in sidebar

When a date popup field is rendered in sidebar, it gets hidden in the back of sidebar column when the style to make the sidebar sticky is applied, date popup has a z-index value of 11 in css.
Is it possible to make the sidebar sticky without setting z-index value to 12?
I don't see any reason to put z-index value in 12 for sidebar column, but I'm not an expert in CSS, so what are the implications of disable that value or setting it up in a lower one?
That value is set in line 1105 of rubik core.css:

/**
 * Sidebar Scroll ====================================================
 */

#content .column-side .column-wrapper {
  padding-bottom: 5px;
}
#content .column-side .column-wrapper.fixed {
  background-color: #FFF;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  -m-box-sizing: border-box;
  box-sizing: border-box;
  display: inline-block;
  position: fixed;
  top: 5px;
  z-index: 12; /* THIS IS THE LINE */
}

.admin-menu #content .column-side .column-wrapper.fixed {
  top: 40px;
}