Did this arrive with 3.5, or did I just miss this before? - Alt clicking on any button will pop out the menu item for it to the sidebar. Alt clicking again will hide the menu.
That’s cool 
neat
Welcome to ZBrush 1.23b (or earlier) 
I can at least remember it from ZBrush 2, but I am sure it was in there from a very early version.
For sure… I do have a faint memory of knowing about it a long time ago, but for some reason I just forgot about it!
yes…important aspect of my UI setup. If no trays are open then alt+clicking a button on your shelf opens the left tray and adds the button’s parent menu.
What I wish was that the left or right trays automatically open if you hover the mouse at the edge of the screen like Photoshop CS4.