This is something I have noticed in plenty of software over the years, but I'm posting here because I'm currently having this issue with Affinity Designer (desktop).
See attached image at full resolution for an example.
I am making a lot of tiled patterns that need to connect seamlessly. I create them with a grid and am careful to snap my points to the grid perfectly. The coordinates in the info panel confirm this. Yet my shapes do not always join and I can see the background between t