The volume and brightness sliders in the control center on iOS and iPadOS also do the slide anywhere thing.
I found it reasonably discoverable.
At first I assumed I had to touch the boundary between the light and dark part of the slider, but eventually I missed and noticed it still slid. I then experimented to see how close I had to get, and found that anywhere in the control would do and once sliding I did not need to stay inside.
One of the really hidden ones in iOS is press-and-hold the on-screen spacebar to move the text cursor around. I found that one entirely by accident, and it’s made text entry a lot less painful.
Interesting, I’ve never consciously thought about where I’m touching to start the drag, I guess it has ”just worked” wherever I have put my finger. The control does not have a ”physical” slider knob so I think it’s fairly intuitive to expect that you don’t have to be particularly precise (like in touch interfaces in general!)
The not needing to stay inside is true of almost all draggable controls on iOS btw. And you can combine that with multitouch. In a music application you might have 4 or 5 parameters under your fingers, nowhere near the original controls which are small and close together. Seems to work everywhere.
I found it reasonably discoverable.
At first I assumed I had to touch the boundary between the light and dark part of the slider, but eventually I missed and noticed it still slid. I then experimented to see how close I had to get, and found that anywhere in the control would do and once sliding I did not need to stay inside.