Device name:

From "Settings > About Chrome"
Application version: Version 69.0.3497.81 (Official Build) (64-bit)
Operating system: OSX

URLs (if applicable): https://www.reddit.com/r/videos, https://embed.ly/docs/explore/oembed?url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DWgLSXLs5ZYc

Steps to reproduce:
(1) Load any nested iframe below or partially below the fold
(2)
(3)

Expected result: Frame should render completely

Actual result: Frame is partially rendering. Chrome only renders either a sliver, or the portion of the frame that was above the fold.

I work on embeds at Embedly, an API used by many large content providers (salesforce, reddit, medium.com, etc)  This is a critical bug that will affect a large surface area of the internet.  Our guess is that lazyload for iframes you have added in this version of Chrome (69) does not work at all for nested iframes, which is a very common practice for caching and other security purposes across the web. (inspect the iframe at https://embed.ly/docs/explore/oembed?url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DWgLSXLs5ZYc for an example of what I mean by a nested iframe)

Fastest way to reproduce is to just visit https://reddit.com/r/videos in Chrome v69 and expand a video by clicking the play button on a reddit link, such that the embed will fill partly below the fold of the page. The iframe will only render the portion that was visible when the button was clicked, and even after scrolling the rest of the frame into the view, it doesn't render. 

Please email me at cs@medium.com if you need to get a hold of me.
 

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