How to blackout your window without drilling / One year update

One problem popped up about one year after having bought the blinds in 2019: the blind won't stay still except at the most closed position. Anywhere above it started to slowly go up by itself.

It is not super serious as it covers the two main use cases: full closed and fully open, but it is a bit worrying.

Also, we've had some mold grow on the window, so it is something to look out for, and add some mold spray as it comes up. The fact that the window is dark makes the mold harder to spot.