Just to clarify the current behavior...
In most cases, if you resize an existing artboard to be taller, it automatically turns into a scrolling artboard. The original height will be the viewport size seen in the preview, and the new height will be the total height of the content that you can scroll around within that viewport.
So for example, in the original screenshot above you may have started with the "Web 1280" artboard size and then resized it taller -- it looks like it's 1280x1024 in the screenshot? Whenever you select anything inside the artboard, you'll see a blue dotted line indicating the viewport height (and in the Preview window or web sharing view, you can use the mousewheel or trackpad to scroll vertically).
Automatically making a scrolling artboard is handy, but unfortunately it's not always what you want. So, in our next update we'll give you options to turn scrolling on or off on any artboard.
In the meantime, if you want a taller artboard to just be tall -- never scroll -- then you can use this workaround:
- Select the artboard tool
- Drag to draw a new artboard as a custom size
- Optional: Type width/height values in the properties panel to get a more precise size
- Locate one of your old artboards that has unwanted scrolling turned on. Marquee-select over the entire artboard to select everything on it, then cut-paste the content onto the new artboard.
Artboards that were drawn by dragging with the Artboard tool will never have scrolling auto turned on. After step 3, you can use Cmd-D to duplicate the blank artboard you created if you have multiple old artboards to fix.
I know that workaround is a bit clunky -- but luckily it's temporary. In our next release, you'll be able to select (or multi-select!) artboards and just turn off scrolling directly.
Hope that helps explain things!