Vertical centering causes contend to be rendered displaced on the current slide - even after reload

Reproduction of issue:

• Have center: 1 in decker.yaml
• Edit *-deck.md while having a browser display the current slide
• Do changes and save *-deck.md in the editor
• The browser shows a slide that is displaced ~200px to the bottom
• Simply reloading in the browser does not resolve the displacement
• Navigating to another slide, reloading and navigating back does, however