Rendering bug on PolyLine

Using PolyLine to create a diamond-shape. On my machine the attached program renders like this: https://i.imgur.com/Q69RNPE.png Right corner pixel is missing, unless the polyline starts/ends at the right corner.

/lib/modules/5.9.0-1-amd64/kernel/drivers/gpu/drm/i915/i915.ko

No problem with nouveau driver.