Any idea why?
By playing around with the version in require('vega-embed@XXX'), it looks like this started happening in version 3.27.0. (c.f. this fork (v3.26.1) (no scrollbars) and this fork (v3.27.0) (scrollbars)).
My guess is this commit has something to do with it.
I’ll send a tweet to Dominik Moritz informing him of this thread.
It’s the little (…) menu with the Save as / Open in Vega Editor links. If the chart is full-width, then the popup menu vega-embed adds will cause the chart to be wider than the page, and hence the scrollbar.
Yes, this was me. I can’t tell from the comments whether you think there is a bug or not. If you think the behavior should be adjusted, please file an issue on GitHub.