On the desktop version of the Roon, you close the volume control and zone selectors by clicking anywhere on the screen outside of the popup. Same for every other popup. But on the iPhone version, volume control and zone selectors can ONLY be closed by clicking again on the same tiny icons you used to open them. Other pop-ups in the iPhone version work on the same logic as the desktop version, but these two pop-ups (the most frequently used in the whole damn app) work on a different logic. Extremely annoying.
Moved to feedback.