Vertically center a content area until it reaches a certain height?

How do I allow a dynamic (changing in height) content area to be vertically centered on the users screen (regardless of screen size), UNTIL it reaches the point where there is only 100px left of free space at the top of the page?

Here's a diagram:

I'm not entirely sure whether the solution to this problem will use only CSS or javascript - so all suggestions are welcome.

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