I’m unsure, but I suspect its the behavior of not constantly calling background – which calls glClear –
try calling clear every frame as a test to isolate? If that works, You might need to then draw into a frame buffer, then next draw, draw the frame buffer you’ve captured, then draw your new lines, and repeat.
Clearing behavior can change subtly depending the renderer settings, if one path clears the depth buffer or just the color, etc etc.