I am working on a project where there is very little space either side of some patio doors — which curtain headings need the least stackback?
If the room is contemporary in style, then consider either an eyelet or wave-effect heading.
Eyelets hang from a pole and take up very little space when the curtains are open. If your pole needs a centre bracket, then make sure it is not too chunky as it will prevent the curtains meeting when they are closed.
A wave-type heading also stacks back neatly and can be made with special heading tape, pin hooks in a buckram-headed curtain, or a dedicated system which includes special track and tape.
Otherwise use a handmade heading — single pleats take up the least space, or double pleats which also fold neatly when opened. Avoid headings like cartridge or goblet pleats which take up a lot of room when drawn back.