I had this problem a few months ago when I tiled my Inlaws bathroom. The solution was to turn the bath around.
The problem is that the bathroom window overhangs the bath. The customer wishes to install a glass shower screen. The customer has suggested that the side of the window is padded out with wood, then clad with ‘No More Ply’ or ‘Backerboard’. It would mean that there would have to be a grout line running down where the backerboard meets the plaster wall.
Then I had an idea, what if I tanked the wall with a Homelux mat. Would this act as a decoupler? And could I then have the grout line where I wanted it?
The reason for the wood is for future occupants of the house to have the option of removing the wood and reinstating the full width of the window.