Regression: camera Y position readback incorrect when in VR

As mentioned in Slack...

mchen [5:53 PM]
I can't recall where exactly, but I think that current master has an issue where the camera position in VR is not getting the correct height value (missing the standing matrix offset?) when you read back its position via getAttribute('position') or .object3D.position?

dmarcos [5:55 PM]
@mchen those issues should be gone

mchen [6:02 PM]
https://codepen.io/machenmusik/pen/oEQbab
1. run Firefox and go to that codepen
2. enter VR and put on your Rift
3. look for the Y label, look down, and notice it's negative

dmarcos [6:03 PM]
It is the rendered height wrong?

mchen [6:03 PM]
the text should be headlocked, as a child of entity with active camera.
both the head sphere and the headlocked text are displayed below where they should be, which is why I had to look down
(the headsphere should be rendered where the HMD is, but since you're inside it, you may only see the nose cone)
the value appears to be off by roughly 1.6 :confused:
https://github.com/aframevr/aframe/pull/3327#issuecomment-360019236
The HMD height assertion in (1) is not correct, as demonstrated by this example.

mchen [6:25 PM]
this version shows controllers, for reference... https://codepen.io/machenmusik/pen/QQJyXV

• A-Frame Version: latest master
• Platform / Device: Windows 10 / Oculus Rift
• Reproducible Code Snippet or URL: https://codepen.io/machenmusik/pen/QQJyXV

Start by reproducing the behavior in the linked CodePen on Firefox with an Oculus Rift, then inspect the camera position readback through getAttribute('position') and object3D.position. Done means the reported Y value and the rendered head sphere and headlocked text use the correct HMD height instead of being offset by roughly 1.6.