Hey @jcor! Welcome to the forum! Rivian has an optional electrochromic glass which will change from opaque to transparent with how much sun there is. for the fixed glass panel I imagine there would be a cover.
The R1S (SUV) roof options are an "electrochromic glass (which turns from opaque to transparent on demand), a fixed glass panel, a two-piece removable composite roof and a standard fixed roof," according to a tweet from the official Rivian twitter account. So far, all four roof options haven't...
The idea of having a fixed glass roof panel without a cover, doesn’t seem to be a good idea. Cold winters equals more heat loss, more heater time, more battery usage. Not to mention noise in inclement weather conditions.
Porsche did something similar with the Taycan. It's got a panoramic roof with no cover. Their roof is made of multiple layers of insulating glass. Not sure if Rivian has done something similar with their glass.
For the first time in a Porsche, low-emissivity thermal insulation glass is used for the panoramic fixed glass roof. Several wafer-thin metal layers are applied to the insulating glass. As the thermal insulation glass allows short-wave radiation to pass through and reflects long-wave infra-red radiation, the vehicle interior will heat up more slowly in summer. This also means that the heat in the interior is retained longer in winter. In combination with the glass tint, it was therefore possible to dispense with a roller blind. An intermediate layer in the glass structure also provides acoustic insulation for the interior.