Interpret \n as newline for MFM

Markdown doesn't generally consider `\n` a newline,
but Misskey does for MFM.

Now we do to for MFM (and not for Markdown) :)

lib/pleroma/formatter.ex

@@ -124,8 +124,8 @@ def mentions_escape(text, options \\ []) do
     end
   end
 
-  def markdown_to_html(text) do
-    Earmark.as_html!(text, %Earmark.Options{compact_output: true})
+  def markdown_to_html(text, opts \\ %{}) do
+    Earmark.as_html!(text, %Earmark.Options{compact_output: true} |> Map.merge(opts))
   end
 
   def html_escape({text, mentions, hashtags}, type) do

lib/pleroma/web/activity_pub/object_validators/article_note_page_validator.ex

@@ -104,9 +104,9 @@ defp remote_mention_resolver(
     end
   end
 
-  # https://github.com/misskey-dev/misskey/pull/8787
-  # Misskey has an awful tendency to drop all custom formatting when it sends remotely
-  # So this basically reprocesses their MFM source
+  # See https://akkoma.dev/FoundKeyGang/FoundKey/issues/343
+  # Misskey/Foundkey drops some of the custom formatting when it sends remotely
+  # So this basically reprocesses the MFM source
   defp fix_misskey_content(
          %{"source" => %{"mediaType" => "text/x.misskeymarkdown", "content" => content}} = object
        )
@@ -121,6 +121,8 @@ defp fix_misskey_content(
     Map.put(object, "content", linked)
   end
 
+  # See https://github.com/misskey-dev/misskey/pull/8787
+  # This is for compatibility with older Misskey instances
   defp fix_misskey_content(%{"_misskey_content" => content} = object) when is_binary(content) do
     mention_handler = fn nick, buffer, opts, acc ->
       remote_mention_resolver(object, nick, buffer, opts, acc)

lib/pleroma/web/common_api/utils.ex

@@ -289,7 +289,7 @@ def format_input(text, "text/html", options) do
 
   def format_input(text, "text/x.misskeymarkdown", options) do
     text
-    |> Formatter.markdown_to_html()
+    |> Formatter.markdown_to_html(%{breaks: true})
     |> MfmParser.Parser.parse()
     |> MfmParser.Encoder.to_html()
     |> Formatter.linkify(options)

test/fixtures/misskey/mfm_x_format.json

@@ -5,7 +5,7 @@
   "summary": null,
   "content": "this does not get replaced",
   "source": {
-    "content": "@akkoma_user @remote_user @full_tag_remote_user@misskey.local.live @oops_not_a_mention linkifylink #dancedance $[jelly mfm goes here] \n\n## aaa",
+    "content": "@akkoma_user @remote_user @full_tag_remote_user@misskey.local.live @oops_not_a_mention linkifylink #dancedance $[jelly mfm goes here] \n\n## aaa\n\nsome text\nnewline",
     "mediaType": "text/x.misskeymarkdown"
   },
   "published": "2022-07-10T15:37:36.368Z",

test/pleroma/web/activity_pub/object_validators/article_note_page_validator_test.exs

@@ -136,6 +136,8 @@ test "a misskey MFM status with a content field should work and be linked", _ do
 
       assert content =~
                "<span class=\"mfm _mfm_jelly_\" style=\"display: inline-block; animation: 1s linear 0s infinite normal both running mfm-rubberBand;\">mfm goes here</span> </p>aaa"
+
+      assert content =~ "some text<br/>newline"
     end
 
     test "a misskey MFM status with a _misskey_content field should work and be linked", _ do