Beyond isolation, this stuff is also used to test (de-)serialisation.
Much-of-a-muchness in this case since it isn't too thoroughly tested,
but best do it anyway.
To fix the user tag control.
I would have done it locally to the user tag control, but it was pissing
me off because I wanted the add button to be aquamarine (as it's closer
to the accents the screen is already using), but the popover on open was
for whatever reason purple and I just want consistency where possible.