On the first run after upgrading, the changelog dialog should be displayed
The dialog is displayed and jumps around a lot. This seems to be related to the fact that k9 is refreshing messages in the background (have a look at the progress bar). I added a gif/video below:

Modify MessageList.java to always display the changelog like below. This happens on normal updates (without below changes), too.
SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = sp.edit();
editor.putInt("ckChangeLog_last_version_code", 27004);
editor.commit();
ChangeLog cl = new ChangeLog(this);
if (cl.isFirstRun()) {
cl.getLogDialog().show();
}
K-9 Mail version: master (de1612f) but has been like this for a while
Android version: LineageOS 16.0
I've noticed that, too. I want to get rid of the dialog and add a "What's new" item to the drawer instead.
Hmm will this drawer item disappear after clicking it or will it always stay there? I feel like it might take away space from the folder list unnecessarily.
Since the settings and folder manager items got sticky, I have to scroll when switching folders. An additional item that is just clicked one single time would be quite annoying to me.
It will disappear once the "what's new" screen has been opened. The screen itself could also contain a setting to never show the "what's new" item in the drawer again.