Make it more likely that the source code editor doesn't stretch beyon…

[meisterT]

…d the screen.

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Example:

[meisterT]

cc @Kevinjil

[Kevinjil]

LGTM. We could also go back to something like calc(80vh - 400px). On e.g. mobile 400px could be a large part of the screen, that's why I originally moved away from it.