The wheel to which I refer? A scrollbar. (Or swipe-ability in the case of touch.) In HaxeFlixel. Google it in the past year. See? It's pretty much just this one person lately asking for it on Reddit.
But they're not the only one over the years, just the most recent.
You might find this thread and Joshua Granick's post which refer to his Scrollbar.hx and seem promising, but it was made with NME/Flash in mind. It's not clear whether this would work well with HaxeFlixel, targeting other platforms like Android. It's certainly not made with the same model in mind, using timers in place of an overridden
update()function. If it would work, it would need forking and one could probably replace Utils.constrain with FlxMath.bound, and use HF's own tweening library.
You might find this flixel-ui issue showing more interest, but also a dead-end in terms of finding something ready-made, even if you're wanting something within the flixel-ui world instead of vanilla HaxeFlixel. Later this question was posted, the end result being a suggestion of a code bounty to move things along. (Although, Gama11 also linked to this code, which had been viewed 193 times as of this writing. Of the things I found this seems the furthest along, but it's now outdated, and at that point it was also incomplete.)
I'm wondering if it's time to simply extend FlxCamera with a new class, FlxUserScrolledCamera, barring a name having more finesse coming to mind. The idea would be, place all your content off-screen, wherever is most convenient. Then, create a new FlxUserScrolledCamera, whose bounds are around your off-screen content, and voila--either the user can swipe at it to scroll around, or use the scroll bars (which auto-hide), depending on the platform. Optionally they could zoom with pinch gestures or a mousewheel. The scroll bars themselves, if any, would be drawn somewhere on-screen, to be viewed not by the camera itself but by the main camera.
To me this represents the simplest solution, but maybe it's too good to be true. If not, I might as well just do it and share the results!