Do you have 'Focus-follows-mouse'- I believe it is called 'Afloat' enabled ?

Afloat. It has a menu bar option in all OS X applications under Window>Focus Follows mouse that is easily accidentally selected.

Suggest you switch it off and see if that resolves the issue.

Thanks Toad-Hall

I can find no such option in any menu bar of any app ie. Thunderbird.... or in the system preferences.

I have found a work around by making the message pane smaller... but that isn't best and I hate "work arounds" when an app should work best.

Dave

I'm not a mac user, but can none of these half-submerged dialog windows be dragged up to show the full display? Clipped or otherwise incorrect dialogs have been seen before with TB on OSX. Does it make any difference if you Help/Restart with Add-ons Disabled (safe mode)? In safe mode, a default set of toolbars and layout are enforced, and graphics acceleration is turned off.

Thanks There was an extension called SmoothWheel (AMO). Disabling it made no difference.

It appears that the dialogue panes are now anchored to the bottom of the message pane.... when I shrink the message pane, the dialogue pane moves up and can be seen. When I enlarge the message pane the dialogue pane moves down to the bottom of the screen and disappears from view. I can't then see or click on the activity buttons.

As I mentioned, the dialogue panes used to be anchored at the top of the message pane and would cover over the text in message. This was not an issue except for the spell check... you need to see the text to decide about the spelling error.

SO either: going from OS 10.12 to OS 10.13 changed the way the OS responds to the TB code to anchor the dialogue panes or TB changed its code about anchoring the dialogue pane to prevent the spell checker from covering the message..... ????

How can I find out which?

I'm afraid I can't offer much advice, and I don't know where such OSX issues are best posed, as there are relatively few TB users on OSX. It seems it would be difficult to revert to 10.12, although perhaps you have another computer with that OS. I might try the simple option of reinstalling TB in 10.13, in case that makes the OS 'reset' it's connection to TB, without any leftover elements of 10.12 (reinstalling doesn't affect the profile folder). It may also be that TB has not been tested sufficiently in 10.13, and the fault lies in TB.

Although I don't think it will make any difference, you could try the TB 60 RC4 version, which is very close to the next release version.