Zulip supports an extended version of Markdown for messages, as well as
some HTML level special behavior. The Zulip help center article on message
formatting is the primary
documentation for Zulip's markup features. This article is currently a
changelog for updates to these features.
The render a message endpoint can be used to get
the current HTML version of any Markdown syntax for message content.
Code blocks
Changes: As of Zulip 4.0 (feature level 33), code blocks
can have a data-code-language attribute attached to the outer HTML
div element, which records the programming language that was selected
for syntax highlighting. This field is used in the
playgrounds feature for code blocks.
Global times
Changes: In Zulip 3.0 (feature level 8), added global time
mentions to supported Markdown message formatting
features.
Image previews
When a Zulip message is sent linking to an uploaded image, Zulip will
generate an image preview element with the following format.
<div class="message_inline_image">
<a href="/user_uploads/path/to/image.png" title="image.png">
<img data-original-dimensions="1920x1080"
src="/user_uploads/thumbnail/path/to/image.png/840x560.webp">
</a>
</div>
If the server has not yet generated thumbnails for the image yet at
the time the message is sent, the img element will be a temporary
loading indicator image and have the image-loading-placeholder
class, which clients can use to identify loading indicators and
replace them with a more native loading indicator element if
desired. For example:
<div class="message_inline_image">
<a href="/user_uploads/path/to/image.png" title="image.png">
<img class="image-loading-placeholder" src="/path/to/spinner.png">
</a>
</div>
Once the server has a working thumbnail, such messages will be updated
via an update_message event, with the rendering_only: true flag
(telling clients not to adjust message edit history), with appropriate
adjusted rendered_content. A client should process those events by
just using the updated rendering. If thumbnailing failed, the same
type of event will edit the message's rendered form to remove the
image preview element, so no special client-side logic should be
required to process such errors.
Note that in the uncommon situation that the thumbnailing system is
backlogged, an individual message containing multiple image previews
may be re-rendered multiple times as each image finishes thumbnailing
and triggers a message update.
Clients are recommended to do the following when processing image
previews:
- If the client would like to control the thumbnail resolution used,
it can replace the final section of the URL (
840x560.webp in the
example above) with the name of its preferred format from the set
of supported formats provided by the server in the
server_thumbnail_formats portion of the register
response. Clients should not make any assumptions about what format
the server will use as the "default" thumbnail resolution, as it may
change over time.
- Download button type elements should provide the original image
(encoded via the
href of the containing a tag).
- Lightbox elements for viewing an image should be designed to
immediately display any already-downloaded thumbnail while fetching
the original-quality image or an appropriate higher-quality
thumbnail from the server, to be transparently swapped in once it is
available. Clients that would like to size the lightbox based on the
size of the original image can use the
data-original-dimensions
attribute, which encodes the dimensions of the original image as
{width}x{height}, to do so. These dimensions are for the image as
rendered, after any EXIF rotation and mirroring has been applied.
- Animated images will have a
data-animated attribute on the img
tag. As detailed in server_thumbnail_formats, both animated and
still images are available for clients to use, depending on their
preference. See, for example, the web
setting
to control whether animated images are autoplayed in the message
feed.
- Clients should not assume that the requested format is the format
that they will receive; in rare cases where the client has an
out-of-date list of
server_thumbnail_formats, the server will
provide an approximation of the client's requested format. Because
of this, clients should not assume that the pixel dimensions or file
format match what they requested.
- No other processing of the URLs is recommended.
Changes: In Zulip 9.0 (feature level 276), added
data-original-dimensions attribute to images that have been
thumbnailed, containing the dimensions of the full-size version of the
image. Thumbnailing itself was reintroduced at feature level 275.
Previously, with the exception of Zulip servers that used the beta
Thumbor-based implementation years ago, all image previews in Zulip
messages were not thumbnailed; the a tag and the img tag would both
point to the original image.
Clients that correctly implement the current API should handle
Thumbor-based older thumbnails correctly, as long as they do not
assume that data-original-dimension is present. Clients should not
assume that messages sent prior to the introduction of thumbnailing
have been re-rendered to use the new format or have thumbnails
available.
Mentions
Changes: In Zulip 9.0 (feature level 247), channel was added
to the supported wildcard options used in the
mentions Markdown message formatting feature.
Spoilers
Changes: In Zulip 3.0 (feature level 15), added
spoilers to supported Markdown message formatting
features.
Removed features
Changes: In Zulip 4.0 (feature level 24), the rarely used !avatar()
and !gravatar() markup syntax, which was never documented and had an
inconsistent syntax, were removed.
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