What did I do about the merry-go-round? Your layout code works with the scroll area code. You tell it whether you intend to FIT_WIDTH or FIT_HEIGHT, and then it tells you whether you can expect a scrollbar, or, in the weird glitchy zone of the merry-go-round, whether to do the opposite of what you normally would, so that it's a bit less glitchy.
There are some off-by-one conversion areas which make the glitchy zone still a tiny bit glitchy, but overall I'm fairly happy with the result. This is a spoiler (if not much of one) from a game whose name is still under wraps, but you'll hear about more soon. These show FIT_HEIGHT, although probably you would typically want FIT_WIDTH, which is more like a web page with a vertical scroll bar.
|Getting too short (you can see the scrollbar almost running out; the "CC" trophy is the last one)|
|Too short for a scrollbar, but too tall to max the height of everything (this would be where the merry-go-round would normally happen)|
|Short and wide enough that we can max the height of everything without a scrollbar|
Input response will be next. For now, you can plunk those .hx files in your own project's source/flixel/addons subdirectory and create a FlxScrollableArea in your state's create() function. This was made and tested with a non-standard scale mode in mind, where you want full control over resizing your content, so the next step is to override onResize() in your state, and there set the .viewPort property of your FlxScrollableArea, re-layout your content based on its .bestMode/horizontalScrollbarHeight/verticalScrollbarWidth properties, then finally set the .content property to match your newly-laid-out content.