YAML metadata causes line number offset for sourcepos

[gpoore]

sourcepos line numbers don't correspond to file line numbers when there is YAML metadata. Metadata introduces an offset equal to the number of lines of YAML plus any whitespace lines that follow.

This is for pandoc.exe 2.17.0.1.

Example file preview_test.md:

Text

Output of pandoc -f commonmark_x+sourcepos -t native -s ./preview_test.md:

Pandoc
  Meta { unMeta = fromList [] }
  [ Div
      ( ""
      , []
      , [ ( "data-pos" , "./preview_test.md@1:1-2:1" ) ]
      )
      [ Para
          [ Span
              ( ""
              , []
              , [ ( "data-pos" , "./preview_test.md@1:1-1:5" ) ]
              )
              [ Str "Text" ]
          ]
      ]
  ]

Modified example:

---
key:
  |
  value
...


Text

Modified output:

Pandoc
  Meta
    { unMeta =
        fromList [ ( "key" , MetaBlocks [ Para [ Str "value" ] ] ) ]
    }
  [ Div
      ( ""
      , []
      , [ ( "data-pos" , "./preview_test.md@1:1-2:1" ) ]
      )
      [ Para
          [ Span
              ( ""
              , []
              , [ ( "data-pos" , "./preview_test.md@1:1-1:5" ) ]
              )
              [ Str "Text" ]
          ]
      ]
  ]

I'm working on a commonmark_x preview extension for VS Code, and so far this has been the only thing interfering with scroll sync between Markdown source and HTML preview. It's possible to try to work around this for now by searching for the ending --- or ... to calculate the offset manually.