Labels don't draw on secondary monitors

[TwinFan]

Describe the bug
the labels only appear for the planes showing on my central monitor (main monitor). Those on the 2 side monitors have no labels, but acquire them as they cross into the middle one.

Also, users consistently report (for ages already) that in VR labels are seen in one eye only. Assuming that the second eye is technically provided as a second screen, this issue is actually the same.

To Reproduce / Source Code Example
Need 2 or monitor setup. Enable Labels.
Watch labels for planes on secondary monitor.

Expected behavior
All planes have labels on all monitors.

Version

• OS: any
• X-Plane: any

Additional context
Reported in Discord's beta channel.

[TwinFan]

"no difference in setting up 2 monitors properly (with lateral offsets etc.) or just mirroring the 1st display: both show no labels on the 2nd display."

[TwinFan]

Investigated: The technique used is currently available on the main screen only:
There is a 2D drawing callback (for phase xplm_Phase_Window, other phases would only be called for 2D cockpits), which uses OpenGL projection matrices to convert 3D coordinates in 2D screen coordinates and then XPLMDrawString for the actual label.
But the actuall call to the callback happens only for the main screen context. From there,

• no drawing to the secondary screen is possible,
• the projection matrices for coordinate conversion for the second screen aren't available.
  I would assume that XP would need to call out callback once per screen to make the above happen, but I can't find a way of doing that.

I comment accordingly to the sample code, in the hope that maybe someone replies there.

[TwinFan]

Ben replied (13d ago already) as follows:

hmmm – if you set the drawing callback to the before windows phase (the example shows after) do you get multi-monitor working? The second eye is not a second screen, it’s a second pass. But the ‘after’ phase isn’t in the HMD at all, the before is.

But changing the callback to "before" doesn't change the situation: It is still called once only, for the main screen