Reproduced this on meta:
// If the editor contains the following text
first line
second line
// Selecting them and clicking the `Hide Details` button would produce:
[details="Summary"]
first line
[/details]
[details="Summary"]
second line
[/details]
// While the desired behaviour should produce:
[details="Summary"]
first line
second line
[/details]
Took the liberty to fix it and added a test:
https://github.com/discourse/discourse/pull/5290
P.S. I did notice @sam’s previous commit, but I believe the { multiline: true } change was unintentional.