Sometimes, after working perfectly well for a while, all of a sudden, lines towards the top-left corner of the screen appear when hovering a shape. The only way to get rid of these is to restart the application

I haven’t been able to find what triggers that but sometimes, lines towards the top-left corner or inside a shape appear when hovering an item (text, shape, whatever).

This is highly disturbing. The only way to get rid of them is to restart the application.