Not possible to place non-projected DOM content in front of projected content in stacking context

[potch]

The following doesn't seem to be possible:

<div class="fullscreen">
  <div class="threed content">
    html/css 3d content
  </div>
  <div class="hud">
    non-3d debuging/HUD info or WebGL content
  </div>
</div>

My assumption is that content that hasn't been given preserve-3d wouldn't be passed through the FOV projection, but it appears that it is:

The debug text in the above image does not have preserve-3d set- it is a direct descendant of the top-level container that was taken fullscreen.