Which links and sites are handled specially (oneboxed) in chat? How do these oneboxes work?

What is the full list of supported and integrated sites?

The current list of integrated (we call this onebox, or oneboxing, ala search engines) sites is:

• Stack Exchange sites: Home pages, questions, answers, users, and comments
  • The source content needs to be able to be seen by an anonymous user (i.e., it needs to not be deleted or on a private beta site).
  • Any hover titles in links contained in the oneboxed content (e.g., those created with [Link title](https://example.com "hover title") will be stripped out.
  • Note that when posting a link to a question or answer from the "Share" button on the post, the system will strip out the referring user ID from the oneboxed link.
  • Due to a known bug, links to answers that are copied from search result pages will not onebox. To ensure you don't run into this bug, click on the link first then copy it either from the browser URL bar or from the "Share" link on the post.
• Stack Exchange Chat: Messages, rooms, user profiles, and bookmarked conversations
  • This works for links from all three chat servers, regardless of the server it's posted on. As an example, links from Stack Overflow chat can onebox even when posted on the general Stack Exchange chat or Meta Stack Exchange chat.
  • This works for all messages in public and gallery rooms. This also works for messages in private rooms if the user posting or editing the message has read access to the private room.
• Area 51 proposals
• Posts from the Stack Overflow blog
• Launchpad bugs
• Man pages, specifically the Ubuntu-hosted versions
• Trello cards (as long as they're publicly readable, of course)
• Wikipedia pages (exception: interwiki URLs)
• XKCD comics
• YouTube videos
• Wiktionary pages (exception: interwiki URLs)

Note that the link must be the only thing in the chat message, after a possibly replying-to link (:123456). In particular, a message containing @user won't onebox.

Are oneboxes updated if the resource behind them is changed or removed?

No. Once created, oneboxes are static, not dynamic. The HTML for the onebox is rendered at the time the chat message containing the onebox is posted or edited and is stored in the database along with the chat message's source text. Later changes to the oneboxed resource will not affect the onebox unless the message is edited, which will trigger the onebox to be re-rendered (or not rendered if the resource that's being oneboxed has been deleted).

For example, if a onebox is created and the thing it links to is later deleted, the onebox will still contain the content that was deleted. Similarly, if the title of a question changes, oneboxes of the question will not be updated. In both cases, the onebox in chat will stay as is. Clicking a link to a comment that has been deleted, for example, will still take you to the post (if it hasn't also been deleted), it just won't scroll to where the comment used to be, unless you have permission to see deleted comments (e.g., you are a moderator on that site or a Community Manager). Links to deleted answers will scroll to the deleted answer, if the user clicking the link has permission to see the deleted answer (e.g., enough reputation to see deleted posts or is the author of the deleted answer).

The same thing also applies if the site from which the content was oneboxed was on the list of supported and integrated sites at the time the message was posted, but was later removed from the list. As an example, prior to May 2019, Twitter tweets could be oneboxed, but support for that and four other sites were then removed for security reasons - messages with oneboxed tweets remain as is unless edited by a moderator, in which case they will revert to being plain links.

The only exception to this rule is any images in the onebox. Images (e.g., user avatars, site logos, and YouTube thumbnails) are hotlinked from the site in question, so any changes to those will be reflected in the onebox. For instance, if a YouTube video is removed or made private, its oneboxes will no longer show its thumbnail and a placeholder thumbnail will show instead. However, the core HTML of the onebox (which includes the video title in the example) will remain static, as above.

All of the above paragraphs also apply even if the message is deleted and later undeleted.