Offset curve issue with closed linestring and miter join

[nyalldawson]

A closed linestring (eg LineString (0 0, 10 0, 10 10, 0 10, 0 0)) gives the correct result when offset by -1 using a round join style:

However if a Miter join style is used, then parts of the result are missing:

Shifting either line endpoint by a tiny amount gives the expected result (eg using LineString (0 0.00000001, 10 0, 10 10, 0 10, 0 0))

[dr-jts]

Interesting. It works fine in JTS:

Possibly related to #1036? Investigation required.

[nyalldawson]

Sorry, I posted my ticket a little early! I should have added that I'm testing using 3.12.1