Good news, there is a fix on beta via bug 1853442 which may help you.You can get the beta for testing from https://www.thunderbird.net/en-US/download/beta/. It can be installed run along side your current version 115.
> By split screen, means that when opening an email, the data is displayed directly below the email headers. Vertical view does not do this.Please refer to the View > Layout for the proper name.> I have downloaded 126.0b2. If I install it over 0b3 wi...