When I use vera/unicode.12.font for either acme or rio (see the
screenshot attached) and I write e.g. 'troff', then I move in front of
'troff', write something there, say 'mv', 'space', and move the mouse
cursor away by clicking e.g. at the end of the line, the 't' letter of
the 'troff' word is not correctly displayed.

What causes this?
Thanks
Ruda