I’ve noticed a problem with Lightning view panel in Thunderbird. After each restart of Thunderbird it disappears and I must resize it using vertical line on the right.
User Skip Head has solved this problem:

hover your mouse pointer over the line, in the middle of line where the tiny
dots are – your pointer will turn into a hand. Click on the dots and
the today pane will jump out to regular size. Now it will stay the
correct size.

Click here for more details