You are talking about the 'Message Pane'. It is not a preview pane nor does it show 'previews'. If the 'Message Pane' is open and you single click on an email in the list, that email is opened and displayed in the 'Message Pane'.

Because you have the 'Message Pane' open, the list of emails area is shorter and there may be more emails listed than can be displayed. In this instance, a scroll bar appears on the right offering another way of seeing the rest of the list.

If you close the 'Message Pane', the amount of room for the list is greater and if all messages in the list can be shown in the available area then there is no need for a scroll bar and so you do not see it. This may explain your situation. To test; Shorten the height of the thunderbird window, then the area may not be big enough and then a scroll bar would auto appear.

I did what you suggested before I wrote. So I know about the window area.

Don't know why, but the scroll bar just automatically showed up again and I touched nothing.

As I sit here looking at my T'bird messages, the scroll bar comes and goes. Makes no sense to me. Once upon a time, it remained constant. Also, if I switch from my mouse to no mouse (meaning my finger), the scroll bar is invisible.