Good job, it’s nice to be able to use the grain line even if it’s not perfect yet, thanks!
I noticed two things while putting grainlines on one of my pattern.
The first thing is not really a bug, in detail mode, the triangles at the ends of the line stay always the same size, so it looks a bit weird when zooming in and out because the line itself gets longer/shorter and thicker/thiner.
The second thing is if we put the length in the dialog greater than the available space, then the grain line is going beyond the limit of the pattern piece, and then you can’t change the length and rotation directly in the detail mode anymore. But you can stil change the values again in the dialog.
Other than this I didn’t notice any bug.