WebGLRenderTarget doesn't seem to work in WebVR

I modified the crossfade example to support WebVR by simply adding an VRButton. But in WebVR, there is no crossfade when the scenes are switched, instead a black screen.
I tried to follow the suggestion in issue 15614 by disabling and re-enabling VR when rendering the buffer scene,but it didn’t work。

How can I fix this problem.