The background attachment fixed ONLY SHEN YOU NEED TO COMBINE IT WITH "BACKGROUND-SIZE: CONTAIN (NOT "COVER)is difficult to understand and this is experimenal only. If there is content above the image (such as nav or whatever) things get weird and the background position px value will have to be adjusted. The min-height must be adjusted for the background image to show correctly esp. in mobile. See code.

Note that if you use "background-size: cover" all you need to do is set the min-height to, for example, so vertical height value, say 50vh and all is well. It is only when cover is not appropriate so that background-size: contain or background-size: 100% must be used that placing the text inside the background-fixed container gets weird.