qmd writer drops empty header row from pipe tables and promotes the first body row to a header

[rundel]

A pipe table whose header row is empty (every header cell blank) is parsed as a Table with TableHead [] [] (zero header rows). The writer omits the empty header line entirely, emitting the alignment delimiter directly above what was the first body row. The re-parser then reads that body row as the table header, so head$rows flips from length 0 to length 1 and the body loses one row.

$ printf -- '|     |     |\n|-----|-----|\n| a   | b   |\n| c   | d   |\n'
|     |     |
|-----|-----|
| a   | b   |
| c   | d   |

$ printf -- '|     |     |\n|-----|-----|\n| a   | b   |\n| c   | d   |\n' | cargo run --bin pampa --
[ Table ( "" , [] , [] ) (Caption Nothing []) [(AlignDefault, ColWidthDefault), (AlignDefault, ColWidthDefault)] (TableHead ( "" , [] , [] ) []) [TableBody ( "" , [] , [] ) (RowHeadColumns 0) [] [Row ( "" , [] , [] ) [Cell ( "" , [] , [] ) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "a"]] , Cell ( "" , [] , [] ) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "b"]] ] , Row ( "" , [] , [] ) [Cell ( "" , [] , [] ) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "c"]] , Cell ( "" , [] , [] ) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "d"]] ] ]] (TableFoot ( "" , [] , [] ) [] ) ]

$ printf -- '|     |     |\n|-----|-----|\n| a   | b   |\n| c   | d   |\n' | cargo run --bin pampa -- -t qmd
| a   | b   |
| --- | --- |
| c   | d   |

$ printf -- '|     |     |\n|-----|-----|\n| a   | b   |\n| c   | d   |\n' | cargo run --bin pampa -- -t qmd | cargo run --bin pampa --
[ Table ( "" , [] , [] ) (Caption Nothing []) [(AlignDefault, ColWidthDefault), (AlignDefault, ColWidthDefault)] (TableHead ( "" , [] , [] ) [Row ( "" , [] , [] ) [Cell ( "" , [] , [] ) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "a"]] , Cell ( "" , [] , [] ) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "b"]] ] ]) [TableBody ( "" , [] , [] ) (RowHeadColumns 0) [] [Row ( "" , [] , [] ) [Cell ( "" , [] , [] ) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "c"]] , Cell ( "" , [] , [] ) AlignDefault (RowSpan 1) (ColSpan 1) [Plain [Str "d"]] ] ]] (TableFoot ( "" , [] , [] ) [] ) ]

Examples from quarto-web:

• https://github.com/quarto-dev/quarto-web/blob/8b1be24df1c9301f2700c77e41e7a9f789535717/docs/authoring/callouts.qmd#L87
• https://github.com/quarto-dev/quarto-web/blob/8b1be24df1c9301f2700c77e41e7a9f789535717/docs/output-formats/all-formats.qmd#L33