I find that Thunderbird will shift focus from the list of emails to the body of one. If one does an up or down arrow fast enough it lets one move through the list but any pause and the focus shifts to the body of the then highlighted email. I see that this was supposed to have been fixed in version 45.1.1 but I have 57.7.0 and it is happening. Is there a setting to make it stop? Of note is that at times the delay before the focus changes is long enough so that it is not a big factor. The delay seems a bit random.