I have an issue (attachment shows a reduced demonstration) where the miters of joined line segments in a PathCollection extend unreasonably far when the angle between them grows acute (use the trackball to rotate the figure in the demo to see the behaviour).

Changing the miter_limit parameter appears to have no effect.

I suspect that the cap and join parameters are necessary to confine the extension of the lines, but I've not found any documentation on what values they take. I haven't yet had time to decipher the complex shader code to figure it out.

I'd be happy to contribute a PR to elucidate this, but first I need to understand it myself!
miters.zip