Bend the hinge pin
If you have a door that swings all the way open every time you leave it slightly ajar, the door frame isn't plumb. But you don't need to take the door down and go through the hassle of disassembling everything just to adjust the frame. Instead, try this much simpler fix.
Grab a hammer, a few scraps of wood, a large nail and a shim. Take them into the room and close the door. Stick the shim loosely between the door and the jamb to hold the door in position when you drive out the upper hinge pin. Then bend the pin slightly with a firm whack. Reinsert the pin and check the results. If the door still won't stay open, do the same with the lower hinge.