To be fair, the last three spoilers become parts of each other in the web UI, so maybe it's ok that those act weird in Thunder, too

one-liner without newline
Content

Text A

one-liner without `:::`
Content

Text B

 spoiler one-liner with newline
Content
:::

Text C

Produces

Text A

Text C

Thanks for the feedback! It would be awesome if you could create a new GitHub issue on this so that it makes it easier to keep track of.

Just as you mentioned, there doesn't seem to be a definition for what correctly defines a spoiler tag in Lemmy, so it's a bit difficult to get all the edge cases right! I kept the parsing to be limited to what Lemmy UI produces when you press the spoiler tag (which adds a space after :::). This is also non-standard markdown which makes it particularly more difficult to implement 😅

For reference, this is what Lemmy UI generates as a template:

spoiler
___