I only found this strange effect after quite a bit of chin stroking and hair pulling with something that was confusing the heck out of me. Workflow to re-create the issue (in Designer for Windows): 1. Create a new document containing an artboard (A5 landscape will be fine). 2. Draw a triangle that almost covers the whole artboard (don't go near the edges so you can see the effect better). 3. Give the triangle a Black stroke colour and a thick stroke width (16pt will be fine). 4. Go