Most GUI-based scorewriter applications allow users to drag items to
arbitrary positions and presumably, creates no side effect. Here, by "no
side effect", I mean the positional alteration of desired items does not
affect the position of any other item on the score, which is like
editing an SVG image in softwares like InkScape.
Paolo Prete has developed the Spontini editor for LilyPond. It allows drag and drop on a web browser, but creates compilable LilyPond code in the file (so you don't need to only use Spontini).
You can find the editor (with a demonstration video) on the github page:
You might find this helpful to accomplish your wish.
Carl