Skybox does not show

My Skybox does not show up in Scene mode.

It showed up before, but recently I don’t know why the problem happens frequently. In these days, it didn’t show up.

I have tried adding the default skybox to Edit->RenderSettings->Skybox Material ,also to MainCamera->Component->Rendering->Skybox.
It still won’t show the skybox

The version is 4.6.9f1

There’s a button in the scene view window that enables the skybox among other things:
69842-skybox.png

Might look different in 4.6 but it should be there somewhere