Bypass or sliding shower doors are a common solution for a small bathroom or bathtub showers.
Gap above shower door.
Different types of weatherstripping are made from rubber felt and foam.
Consist of two and sometimes three panels that slide along each other on tracks.
Tear off the tile of the existing wall in question.
Raised decorative metal panels that are designed to look.
Decorative panels intended for use as backsplashes on kitchen and bathroom sinks can be installed above the shower surround.
Some are nailed on some have adhesive backings and some slide into a notch in the door jamb.
A small gap above the door can be filled by weatherstripping which is flexible and will conform to the gap filling the space up and preventing air from entering and exiting.
The one i like twyford t4 or geo6 is 1 9m.
The distance between the tray and the ceiling is 1 96m.
Sliding door operation requires no floor space for opening door.
There is an extractor in the ceiling about 50cm away from where the door will be.
Bypass or sliding door dimensions alcove shower bathtubs commonly are 60 inches wide and their bypass or sliding doors are usually about 58 inches.
The door guy said 24 inches is a good number depending of course on your ceiling height.
Shower enclosure seem to come in 1 85m 1 9m and 1 95m.
It must be exactly 60 inches.
The industry standard is 1 16 but i think even that is pushing it.
I think it is reasonable to reject the 1 4 difference in size of the gap between the door and fixed panel.
When framing the alcove do not add any extra space to accommodate the shower tub.
Ideal for wide openings with a typical opening of 60 inches wide.
Question is will it be a problem only having a 6cm air gap above the door and side panel.
Then build it out with 1 2 hardee board or durock and then cover it with tile.
The edge seal along that edge is probably there to hide the condition you are describing.