A table's omitted outer pipes are added on save

[Azganoth]

Summary

A table authored without leading and trailing pipes is written back with them, so a file using GFM's pipe-optional form comes back in the form the serializer prefers. Both forms are the same table to a GFM reader, so nothing about the document changes.

This is the half of #323 that Preserve the form a file was written in keeps rather than normalizes. Whether a table carries outer pipes is one property of the table, not a layout computed across its cells, and it survives an edit to any cell, which is what separates it from the padding that decision normalizes.

Steps to reproduce

1. Open a document containing a table whose rows omit their leading and trailing pipes.
2. Save it without editing.
3. Read the file.

Expected behavior

A table authored without outer pipes is written back without them, and one authored with them keeps them. A table created in the editor writes the form defined as its default.

Actual behavior

Measured in #323 against 83e12b19 by driving the editor mount used by the plugin tests. From corpus/gfm/tables.md, this table:

Bed | Reading | Action
:--- | :-----: | -----:
North | 31% | Move
South | 42% | Hold

is written back with an outer pipe on both sides of every row.

Related context

• Related issues or pull requests: A header-only table gains an empty body row #323 measured this and keeps the header-only body row, which is a separate defect in the schema rather than the serializer. Every thematic break is rewritten as three asterisks on save #319 preserved a thematic break's authored run by the technique this needs. Saving pads a ragged table row into new cells #254 fixed ragged rows being padded into new cells and left this whitespace out of its scope.
• Documentation, decisions, or prior investigation: Preserve the form a file was written in in docs/decisions.md; corpus/gfm/tables.md; Preserve the authored form of a Markdown file on save #251 under Scope.

Done when

• A table authored without outer pipes is written back without them.
• A table authored with outer pipes keeps them.
• A table created in the editor writes its defined default form.
• A table whose first or last cell is empty, or whose first delimiter cell would be a single hyphen, is written with its outer pipes whatever form it was authored in.
• Regression coverage asserts the file rather than only the document.

Notes, logs, screenshots

Implementation direction

markdown-table already supports this through its delimiterStart and delimiterEnd options, but mdast-util-gfm-table@2.0.0 never passes them: its handleTable calls markdownTable with align, alignDelimiters, padding, and stringLength only, and exposes just tableCellPadding, tablePipeAlign, and stringLength as settings. Reaching the option therefore needs a local table handler rather than a serializer setting, and the authored form has to be carried on the table node for that handler to read. src/features/editor/plugins/tableShape.ts is the existing local seam.

The default an editor-made table writes is not defined yet. #319 settled the equivalent for a thematic break by naming *** in docs/decisions.md and docs/specification.md. The same choice is open here, and a table the editor creates carries outer pipes today because the serializer writes them for every table, not because the form was chosen.

That default is also the fallback, because omitting the outer pipes is not always safe. Measured against markdown-table@3.0.4 and micromark-extension-gfm@3.0.0 by rendering a table with delimiterStart and delimiterEnd off and reparsing the result:

• A row whose first or last cell is empty stops being a table. | B and A | are not read back as the rows they were written from.
• A delimiter row whose first cell is a single - stops being a table, because - opens a bullet list item where no pipe precedes it. This reaches any table whose first column is one character wide, since the delimiter cell follows the column width.
• An empty cell between two filled ones is safe.

So a recorded bare form gives way to the default wherever the first or last cell of any row is empty, or the first delimiter cell would be a single hyphen.

Out of scope

• Cell padding, delimiter row width, and the redistribution of padding inside a cell an alignment marker applies. Preserve the form a file was written in normalizes all three, so they are not defects.
• The empty body row a header-only table gains, which stays on A header-only table gains an empty body row #323.
• Ragged body rows padded or truncated into new cells, fixed by Saving pads a ragged table row into new cells #254.