I think the form trick via 'autoScroll' should be the right Solution because there is a shortcut for autoScroll beside quick layout selector.
As a workaround, promoting nested form to class and selecting form component with autoScroll true allows scrolling inside form area, but always linking and unlinking is not quite comfortable.
The problem existing for nested form panels directly inside viewport (or maybe also deeper) with or without layout: 'fit'.
For viewports with layout: 'auto' and selecting autoscroll: true on viewport it is also possible to scroll down the whole area (but after editing i have to restore all properties to needed export values!).
You can select the component in the Component Tree - it should scroll the form to show the field. When a Viewport is "fit", then autoScroll will mean nothing on it (cuz it's never overflowing, it's fitting its first child into its box perfectly). autoScroll on the FormPanel is what's necessary to create an overflow.
The scrollbar on the Canvas is definitely kludgey w/ the Viewport due to z-index issues. The canvas problems are being addressed in Designer 1.2.
yes I'm aware, that is the z-index issue I mentioned before. But, clicking on a component under FormPanel will cause FormPanel to scroll down. Try it: add a ton of form fields to the formpanel, and then click on one towards the bottom.