Attached is a patch that adds vertically centered fold tail markers. This way you can choose markers that are symmetrical to the fold header markers, which are drawn centered.
Current fold tail markers are not positioned at the center when line height is larger than fold margin width, which can look odd since in that case tail will be drawn in the upper part of the line.
I doubt there was an intention for the original outline markers not to be centred vertically.
I wanted to change existing markers, but as I could not determine if current positioning was intentional, I opted to create separate markers. However, if it is ok with you, I can create a different patch that changes existing markers instead.
And while we're on this subject, is there a reason why square tail marker is drawn 2px from the right, while curved is drawn just 1px from the right? Can the square tail marker be changed so it is drawn 1px from the right like curved one?
Patch that changes existing markers
I've attached a new patch that changes existing markers, so they are centered vertically. The width of square tail marker has also been changed so it is the same as the width of curved one.
IIRC, the initial square tails were chosen to be less prominent. The curved tails were then extended as they looked incomplete in normal sizes.
Committed.