And mouse pointer won't come up with nothing
Will check.
The -d cursor
client log output would be good to have, just when you move over the window and it goes blank.
It is possible that this is a problem with your local theme, in which case this will help:
XPRA_USE_LOCAL_CURSORS=0 xpra attach ...
I found a small core during the experiment. Maybe it is useful in some part?
NameError: name 'ss' is not defined
That's already been fixed in r25193.
I even re-issued 3.0.6 with this fix on top, the problem is that every build succeeded (including xenial 32-bit) except for xenial 64-bit! (which is what you're using).