This seems to be a change since installing NVDA 2020-3 rc1 on my win 10 system with latestest version of TB.  Normally when I open a Email in TB I can use the up and down arrow keys to read a message.  Now it is very incosistent with the arrow keys seemingly to do nothing.  A restart of NVDA solves the problem but keeps coming back to the problem on a random basis.