Alright 
.
I tested, and if oneboxes don’t work well (like, not showing media), then you must add twitter.com in blocked onebox domains in order for the component to work. 
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I didn’t use the theme component widely and didn’t encounter the issue, but my guess is that Discourse doesn’t know how tall will be a Twitter iframe before the content is loaded. When the iframe loads, it pushes the content arround it, creating this behavior.
Can you capture a video of it?