Hi Gjac, You have a very good point.This is what I thought initially since the gap between the door and the frame did effectively changed over time without me being able to relate it to a specific cause.I thought about the leveling but I got stuck with the idea that since the shower tub was made of one piece it could not be influenced by mouvements of the frame. But in reality, since it is fixed to the floor it should effectively reflect any mouvement of the latter. In this light I will monitor it more attentively and let you know eventually.
Thanks for this advice.