Hey there guys,
Such an amazing community we have here. 
Your contribution is highly appreciated in this issue.
While developing one of our themes we had to create a section with position: fixed; right: 0px; the element disappeared far far away to the right of the viewport.
We noticed that the Preview Bar iframe is taking a width of 7104px while the viewport width was 1776px, would be more reasonable make sure to force the iframe width to be 100vw instead of 100%.
Can someone correct me if we are wrong?