The Z order determines the order in which UI elements are shown on top when they overlap. Currently the way to control Z order is:
For UI Containers that overlap, the container which was added most recently to the hierarchy gets shown on top. Otherwise, there is no way to control the Z order, which is problematic.
The Z order of elements within a UI Panel is determined by their order in the panel hierarchy. Elements at the bottom of the hierarchy are shown on top. This is not ideal, but it works in a crunch.
Add a Z order property, which is just a numerical value, that determines the ordering of UI elements. Default is zero for all elements. The developer can adjust the values to get the desired overlap behavior.