I did a little more testing, this time with a VMWare Sierra VM configured as a retina VM.
This one actually draws things in full screen but shows some major scaling problems.
Here is a summary of my findings.
El Capitan (VM):
- desktop correctly reported as 1440 x 900
- if you create a window 1280 x 720, the renderer output size is correctly reported as 2560 x 1440. Draws correctly.
- in full screen mode, the renderer output size is correctly reported as 2880 x 1800. Draws correctly.
Sierra (VM):
- desktop correctly reported as 1440 x 900
- if you create a window 1280 x 720, the renderer output size is correctly reported as 2560 x 1440. Incorrectly scales up textures to double the width and height they should be, meaning you only see the bottom left corner of the texture.
- in full screen mode, the renderer output size is wrongly reported as 5760 x 3600. Draws "correctly" insofar as if you want to fill the screen entirely with a texture it will scale correctly to 5760 x 3600.
Sierra (Physical MacBook Pro):
- desktop correctly reported as 1440 x 900
- if you create a window 1280 x 720, the renderer output size is correctly reported as 2560 x 1440. Draws correctly.
- in full screen mode, the renderer output is correctly reported as 2880 x 1800. Doesn't draw properly at all, renderer seems to like the colour red. After exiting, the display is often unstable and a reboot is necessary.
Possibly the Sierra (VM) results are spurious and caused by something in VMWare's retina implementation. Then again, the El Capitan (VM) works fine so...