Text over bar lines.
Open a piano score, and add a staff text sufficiently long so as to exceed the width of a single bar. Place it between the staves so that the text overlaps the bar line.
In most printed scores I've seen, the barline is broken just enough so as not to interfere with the text. I think Musescore should emulate this behaviour. This might be as simple as having a background around the staff text that is the same color as the page. I can't think of any situations where you wouldn't want the text to be on top of everything else.