https://trello.com/c/3jmb9Ucx/

There is a library for this, jquery.byteLimit.

Unlike maxlength, it is a byte limit, not a character limit, which probably fits the situation here.

(In reply to Matthew Flaschen from comment #2)

There is a library for this, jquery.byteLimit.

This is in core.

In Firebug, a long title has a data-title attribute on the h2 class that is max 200 characters or 199 characters ending with "...".

flow-board.js' editTopicTitle() function passes 'content': $title.data( 'title' ) to the flow_edit_topic_title form template, so this truncated title is what is shown in the form, hence this bug. I think Flow uses this data-title rather than directly using the h2 contents to avoid bugs with improper escaping.

The data-title attribute is only 199-200 characters because handlebars/flow_topic_titlebar_content.handlebars template has

<h2 class="flow-topic-title" data-title="{{plaintextSnippet content.format content.content}}

and the plainTextSnippet() helper function truncates.

We should be using constants for this, even sending them to JS using ResourceLoaderConfigVars. BTW there are two places where substr( 0, 200 ) appears in flow-board.js. The server-imposed length limit comes from PostRevision::MAX_TOPIC_LENGTH = 260. MediaWiki's 255-character title length is discussed in https://www.mediawiki.org/wiki/Page_title_size_limitations and it might be worth matching it for consistency, but a topic title is not the same as a page title. If we do want Flow titles to play by the same rules then we could feed them into a MediaWikiTitleCodec for validation.

Change 166559 had a related patch set uploaded by Matthias Mullie:
(bug 71584) Fetch title content from API

https://gerrit.wikimedia.org/r/166559

Change 166559 merged by jenkins-bot:
(bug 71584) Fetch title content from API

https://gerrit.wikimedia.org/r/166559

Is this fixed?

Tested at http://en.wikipedia.beta.wmflabs.org/wiki/Topic:Sc42mtgxr73cuk4r
Started with a 256 character title. It let me add 4 characters, and saved successfully with all 260 characters.

So, the bug-aspect is fixed.
BUT, the 2 side-notes in the description are still unresolved. Should they be split into 1 or 2 new bugs, or solved as part of this one?

In T73584#1061208, @Mattflaschen wrote:

The titles should be limited to 255 bytes (Not the current 260 bytes). Possibly there's a global variable we can share?)

Is this really worth "fixing" if everything is working fine with 260?

I'm not sure. I'd lean towards yes, for A) Consistency, and B) in case it is documented anywhere, then we don't have to add an exception for Flow... Hmm, I can't see it mentioned in https://meta.wikimedia.org/wiki/Help:Talk_page - perhaps it's fine as is...

Change 192498 had a related patch set uploaded (by Mattflaschen):
Don't trim plaintextSnippet

https://gerrit.wikimedia.org/r/192498

Patch-For-Review

Change 192498 merged by jenkins-bot:
Eliminate plaintextSnippet

https://gerrit.wikimedia.org/r/192498