Filing this following the discussion in T265094.

@ppelberg @Whatamidoing-WMF Do you think this is something we could easily add to the signature requirements? (It's easy to implement.)

Here's a few examples, caught by dtcheck.toolforge.org:

• en.wikipedia.org: user adding a signature with a line break – a later edit has a dirty diff because of it
• simple.wikipedia.org: user adding a signature with a line break – a later edit has a dirty diff because of it
• ar.wikipedia.org: user adding a signature with a line break – a later edit has a dirty diff because of it

FWIW, Parsoid can now handle line breaks for the enwiki and simplewiki cases, but doesn't yet have handling for the arwiki signature because it the link with the line break is nested in ^... While we could add handling for generalized nesting (since it is technically not an error), it just adds extra code complexity with diminishing returns. Some future code cleanup / refactor might bring in support for this more naturally, but that is not happening any time soon.

Change 656973 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[mediawiki/core@master] Check for line breaks in user signatures

https://gerrit.wikimedia.org/r/656973

This seems sensible and uncontroversial:

• they cause problems
• they add no real value
• they are extremely rare
• it is already implied that you aren’t supposed to use them because the signature input is single line

T230653: Use a parser function to encapsulate signatures would mitigate this long-term.

In T272322#6756370, @matmarex wrote:

Filing this following the discussion in T265094.

@ppelberg @Whatamidoing-WMF Do you think this is something we could easily add to the signature requirements? (It's easy to implement.)

Here's a few examples, caught by dtcheck.toolforge.org:

• en.wikipedia.org: user adding a signature with a line break – a later edit has a dirty diff because of it
• simple.wikipedia.org: user adding a signature with a line break – a later edit has a dirty diff because of it
• ar.wikipedia.org: user adding a signature with a line break – a later edit has a dirty diff because of it

As we discussed during the team's 20-Jan meeting, before we can move forward with implementing the line break signature requirement, we'll need to consult with volunteers to learn in what – if any – cases this requirement could be problematic.

This consultation will happen in T272667.

A few other notes from that meeting are here:

• Mass message does break signatures into separate lines: "line 1" contains the sender's username and "line 2" contains the time at which the mass message was sent. However, this does NOT seem to conflict with the new requirement @matmarex is proposing above considering this requirement is limited to cases where people are signing with ~~~~ [i].

i. Note: Mass message "senders" manually type out the contents of "line 1" and type ~~~~~ to produce the timestamp in "line 2".

Change 656973 merged by jenkins-bot:

[mediawiki/core@master] Check for line breaks in user signatures

https://gerrit.wikimedia.org/r/656973