Tree control in a wizard: scroll page or scroll wizard content

mockup

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Context:

  • Desktop web app
  • Wizard (multi-page form) with three steps
    • Appears in modal dialog
  • First step has only one control: tree selector
    • Tree does not fit on screen
  • Two buttons at the bottom of the form: [Next>] and [Cancel]

Since the tree does not fit in the screen, would it be better to:

a) scroll the entire page - as a consequence you won't see the buttons on the form without scrolling - OR -

b) scroll the control and always keep the buttons on screen?

There's a debate between a colleague and myself. He prefers a) using the argument "people scroll" -- there are several articles online arguing for the point. However, I feel that 1) scrolling is more suited for web pages (consuming content) rather than web apps and 2) in this particular case showing all the content at once (including buttons) makes the UI more understandable at a glance. Design choice a) makes the user wonder what lies below the fold.

Opinions?

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