Mouse focus problem in 15.10 - xfwm4 ?

[Chris Green]

I am having mouse focus problems on my laptop running xubuntu 15.10.

Every so often (and it seems to be happening more frequently recently)
the mouse pointer 'sticks' in one mode, not always the same mode though.

For example sometimes it gets stuck in 'character pointer' mode, the
'I' like cursor.  Everything works as normal in the terminal window
which 'owns' the cursor but, although it moves all over the screen,
the pointer stays as a cursor and doesn't do anything when clicked.

Another time the pointer stuck as an arrow and would only work in the
workspace switcher though, as always, I could move the pointer all
over the screen.

I *think* restarting xfwm4 fixes it but I'm not absolutely sure yet. 

It's possible that some wierd setting has got misconfigured though I
have looked through the mouse and window manager settings and can't
see anything particularly wrong.

I've searched and found some fairly old (i.e. older xubuntu versions)
reports of a similar problem but there doesn't seem to be any
particular fix - except that of restarting the window manager.

Does anyone have any ideas what might be wrong?

-- 
Chris Green