For better or worse the spacebar has been the standard keyboard shortcut for scrolling webpages for as long as I’ve been using the web.
When viewing an observable notebook, it starts out working as expected, but if I click on (or select text in, etc.) the rendered output of a cell (which might just contain paragraphs of text, nothing fancy), the spacebar stops working for scrolling.
If I click to show and then hide a cell’s code box, the spacebar starts to work again, but as soon as I click into the content area it breaks.
I can’t imagine any reason to desire this behavior, and it’s pretty annoying, so I am guessing it is a bug.