The problem appear even on a fresh install. Here are some screenshot to describe the problem.

Image a : The window is large enough, the message is correctly displayed. Image b : The window is not large enough, the message is truncated. No scrollbars appears.

Just tested to understand the issue you describe.. Findings: I can see that as I slowly make the window narrower or increase the width of the Thread Pane (mid section), the vertical scroll bars on far right of Message Pane disappear off screen so preventing scrolling downwards.

There is no problem where the the Message Pane buttons 'Reply, Forward, Archive,Junk, Delete, More' occupy full width, but if you reduce width a bit further then the issue arises. So if all those Message header area buttons are in full view then you also get the scrollbar.

If there is a horizontal scrollbar because there is sufficient text, then even if you use it to scroll to the right, there is a point at which it stops and does not reveal all text nor display a vertical scroll bar.

If it is a plain text message, then the text does wrap to window width up to a point;. By which time it really has become quite narrow, so difficult to see why anyone would reduce it that far.

But seems less able with HTML messages that contain html css code controlling the viewable layout. It really depends upon the css code in individual emails as it does not effect some as bad as others.

I've established that this occurs no matter whether you use Wide, Classic or Vertical view I'm going to report this as a bug to see what the reponse is from the developers.

If you wish to follow this bug report then here is the link: https://bugzilla.mozilla.org/show_bug.cgi?id=1670934

Could you please tell me what OS you are using and also what version of Thunderbird as this information will be hepful for the developers.

UPDATE: Sorry, I just realised you had already informed me it was Linux Debian and using 78*.

Thanks for your help Toad-Hall. I have also deduce that the problem is related to the width of message toolbar (which can't be put on 2 lines). I have already open a bug report you can merge with your own : https://bugzilla.mozilla.org/show_bug.cgi?id=1670862 In this bug report I suggest a workaround : use an extension to reduce the size of message toolbar buttons.

You used to be able to 'customise' that Message Header toolbar and they removed it. So you used to be able to eg: only have icons. There has been quite a bug topic on this and I've been trying to get it back but it's an uphill battle. https://bugzilla.mozilla.org/show_bug.cgi?id=1556261

The Message Header buttons can be hidden with a userChrome.css or the add-on mentioned in the bug report, which offers more options, or the Compact Headers add-on, which has the option to replace the buttons with the message recipient.