See the post title, really. In a part layout, in Staff Spacing mode, I can’t click on the handle corresponding to a Tacet to select it. What does work, however, is clicking on an adjacent handle and navigating to it with the arrow keys, or including it in a marquee selection. It can then be moved with (Shift-)Alt-up/down as normal but not dragged with the mouse.
Thanks for reporting this. I daresay it has ever been thus, but I’ve made a note of it and I will look into it when I get a chance.