Add a text fallback for the embed iframe

[pento]

Suggested by @helen:

I would like to see a text fallback (e.g. a link) provided in the oEmbed response, much like the way Twitter includes a blockquote. I wouldn’t want to see more text from the site, but the title/site name would probably suffice.

This can be included in the oEmbed response data, then hidden when we get a message from the iframe to signal that it loaded correctly.

[pento]

34462.diff​ is a first pass at this.

I don't really like the flash of fallback text that appears before it's hidden, but this is the same behaviour you see with Twitter.

[swissspidy]

How would that look like when JavaScript is disabled? I guess the iframe needs to be hidden initially. That would work well with #34451 in mind too.

[swissspidy]

Also, the if ( empty( $content[1] ) && empty( $content[2] ) ) check means a <blockquote> is required, right? I don't think it should be. If we use blockquotes, it doesn't mean every other oEmbed provider does as well.

[pento]

in 34462.2.diff​:

• Hide the iframe by default (if a blockquote fallback is present), then display it once it sends a message, hiding the blockquote at the same time. This causes a bit less jumping around.
• Require an iframe to be present, instead of previously requiring either a blockquote or an iframe.
• Add some unit tests.

[pento]

In 35437:

Embeds: Provide a cached text fallback.

Sometimes, embedded sites might suffer from less than 100% uptime. Instead of leaving the embedding site with a big blank space where the embed should be, let's fall back to a link to the embedded post, so there's at least some context for the post.

Fixes #34462.

[swissspidy]

You'll never receive a postMessage from an iframe that is hidden using display: none.

I'll try to fix this together with #34451.

[pento]

Replying to swissspidy:

You'll never receive a postMessage from an iframe that is hidden using display: none.

Which browser did you test this on? It works fine for me in Chrome, Firefox and Safari.

[swissspidy]

Replying to pento:

Replying to swissspidy:

You'll never receive a postMessage from an iframe that is hidden using display: none.

Which browser did you test this on? It works fine for me in Chrome, Firefox and Safari.

Chrome. I just tested this again on trunk and couldn't reproduce it, so I guess it was something I introduced myself while working on #34451. So, sorry for acting too hastily.