Perhaps I shouldn't have used the term 'Hatari UI' in my original post. That seems to be another, different component :-)
I just meant Hatari itself (v1.8.0) when launched on its own, sometimes does not clean up its window when quit. This is without using the python Hatari UI - just plain Hatari installed via apt-get and launched from the console with 'hatari'.
It happened again today so it's not cured, as I thought. It may just be inconsistent.
When this occurs (either by using the QUIT button, or hitting the red window X), the 'hatari' process is terminated and no longer in the process list. The window however retains the last image shown (usually the TOS desktop or SDL UI) and can be dragged around and minimised as if its still alive. It can't however be closed.