Sticky table header may not consistently remain at the top of the table.

The original column headers show on top of the sticky header, when they should rather be underneath the latter.
Also, elements inside tables with position: absolute; show on top of sticky headers, which they shouldn’t. An example are CTools’ drop-down menus.

Both could be easily fixed by adding this to Drupal’s system.base.css:

table.sticky-header {
  z-index: 1;
}