The default font must be honored, not assumed, when computing column widths

Cluster: styles

Scenario

A workbook's default (normal-style) font is not Calibri 11, say Calibri 72, or a different face entirely. Column widths in the spreadsheet format are expressed in character units of the Maximum Digit Width of the workbook's default font, so the pixel width of a column depends on what that default font actually is. A library that hardcodes an assumed Calibri-11 metric when reading or writing column widths miscomputes them for any workbook whose real default font differs: columns come out too narrow or too wide, and a load-modify-save of such a template visibly breaks its layout.

Desired behavior

• The width model must be anchored to the workbook's declared default font, the normal cell-style font in styles.xml, not to a baked-in Calibri-11 assumption.
• The library must not unconditionally inject its own assumed default font ahead of, or in place of, the default font a file already declares. Reading a workbook whose default font is non-standard and writing it back unmodified must preserve that default font.
• Column width definitions and the sheetFormatPr baseline (defaultRowHeight, defaultColWidth, baseColWidth) must survive a read-write round-trip unchanged.
• Per-cell and per-column style-index references must stay consistent across the round-trip, so styled columns still point at their intended styles.

Prior art / format facts

• OOXML column width is width = (charCount * MDW + 5) / MDW rounded, where MDW is the Maximum Digit Width in pixels of the workbook default font at the workbook default size. Change the default font and every character-unit width changes meaning.
• sheetFormatPr@defaultColWidth, baseColWidth and the default row height are likewise tied to the default font metrics.

Open questions

• How do we represent "the workbook default font" as a first-class, typed part of the model so it is neither lost nor silently overwritten on write? Answered (ADR-0025): Workbook carries declaredDefaultFont (what a file stated), setDefaultFont (authoring), and defaultFont (the resolved, complete result the writer emits as font 0). With nothing authored a file's own font 0 passes through verbatim, so a declared face is never overwritten. Locked by the workbook-default-font-is-declared-not-assumed corpus case.

• Still open, and deliberately so: do we measure MDW from actual font metrics, which requires font metric data for common faces, or carry width as an opaque round-trippable value and only interpret it when a caller asks for pixels? Round-tripping the raw value avoids the metric problem for the common open-modify-save path; true pixel-accurate layout needs the metrics.

  Widths stay opaque for now. Fixing the default font changed what the units mean without changing the units, and read-write stays faithful as long as we neither reinterpret nor recompute them, so nothing about that fix forces the metric question. It is only forced by a caller who wants pixels, or by autoFit.

• Still open, and downstream of the above: what is the fallback when a declared default font is unknown to our metric table? Assume a documented default while preserving the declared font name for re-emission? Moot until there is a metric table.