I am running TB 66.0b2 on MAC Mojave 10.14.3. Prior to beta 2, whenever I filed or deleted an open message, the tab in which it was displayed closed. Now, when I file or delete an open message, it just sits there. I went into Preferences > Display > Advanced and noted that the check box "Close message window/tab on move or delete" was not checked. I checked it and restarted TB. Now the box remains checked, but the function does not seem to work. This is a minor annoyance, but it is an annoyance.