[MlMt] Where have my scrollbars gone?

[Paul Sture]

I've just recently noticed that scrollbar behaviour in Sierra seems to 
have changed.

I can no longer see any down/up icons at the top and bottom respectively 
of vertical scrollbars.

While this applies to all applications, it's a disaster for MailMate.

a) The only way I've found so far to get the vertical scrollbar to 
appear while I am focussed
    on the body of an email is to hit up-arrow or down-arrow.
b) in a MailMate reply to an HTML message, which triggered the Preview 
pane, I was unable
    to scroll back up to text I was trying to answer to.
c) I am currently unable to scroll down through the message summary pane 
- I wanted to select
    a bunch a messages to delete, but couldn't find a way of moving any 
further than the
    bottom of a window.

My mouse's scroll wheel seems to have been disabled.

I am also seeing weird scroll wheel behaviour in Terminal.

Are Apple trying to force me to buy a Trackpad?