Preserve drawing shapes (autoshapes, text boxes) on round-trip

Cluster: images

Scenario

A user starts from a template workbook containing drawing shapes: autoshapes, text boxes, connectors, and other vector graphics anchored to the sheet, distinct from raster images. They edit cell values and write it back. On reopening, all cell data survives but every shape has vanished: the drawing layer the library does not model is silently dropped during read, modify and write. Content the user never touched should survive a round-trip unchanged.

Spec note, not a corpus case: the library has no shape model at all, so this is a feature-gap and data-loss-prevention requirement rather than a discrete assertable bug. The durable value is the preservation contract, and the reconciliation hazard where a drawing mixes modeled images with unmodeled shapes. It belongs with the package-part-preservation family (comments, VML, tables).

Desired behavior

Two levels of ambition:

1. Preservation (minimum, higher value). Drawing parts, their shapes (xdr:sp autoshapes, text boxes, connectors), and associated rels and media are treated as opaque pass-through when the library does not fully model them, so untouched content round-trips structurally intact. This aligns with the fork's "don't silently destroy user data" stance. Care at the mixed-content edge: when a drawing part mixes modeled images with unmodeled shapes, the writer must merge regenerated image anchors with preserved shape anchors rather than choose one wholesale. Otherwise regenerating the drawing from the image model alone drops the shapes, which is the current failure, or preserving verbatim drops newly-added images.
2. First-class modeling (stretch). A real API to create and edit shapes as siblings of images in the drawing collection. The concrete authoring shape, from a second more detailed report: addShape(shapeSpec, rangeOrAnchor, { hyperlink, tooltip }) where the spec carries a preset geometry (prstGeom presets: roundRect, rect, ellipse, arrows, …), an anchor, one- or two-cell, a rotation, a solid fill (RGB, later scRGB or theme), an outline (ln: weight, color, dash pattern such as sysDash), and a text body, an ordered list of paragraphs each with a horizontal alignment and a list of runs (plain text, or text plus font: bold, italic, size, color), plus the box's vertical alignment. OOXML mapping: shapes are <xdr:sp> in a one- or two-cell anchor, parallel to <xdr:pic>; spPr holds xfrm, prstGeom, solidFill and ln, txBody holds bodyPr (vertical align) plus a:p paragraphs (a:pPr align, a:r runs with a:rPr and a:t), and nvSpPr carries name, id and an optional hyperlink. Reuse the existing image-anchor and color/theme vocabulary rather than a shape-local one. This is a large surface, so verbatim preservation (level 1) is the safer near-term win, with typed modeling layered on where fidelity is guaranteed. Unknown geometry presets, unsupported fill and line types, and unmodeled run properties must degrade predictably, preferring verbatim passthrough, rather than fail the load.

Prior art

A long-standing, repeatedly-reported gap hit independently by template-driven workflows. It co-occurs with the general "foreign-generator content dropped on round-trip" family the fork already tracks via package-part preservation (comments, VML, tables, pivots).

Open questions

• Opt-in or default? The fork principle argues default-preserve for content the user did not touch.
• Mixed drawing part reconciliation: anchor-level merge, or whole-part passthrough when any unmodeled element is present?
• Fidelity contract for preserved parts: byte-identical, or structurally-equivalent-after-normalization?
• Do shape anchoring semantics (twoCell, oneCell, absolute) need to be understood even for pure passthrough, or can offsets be preserved verbatim?

Related: roundtrip-preserves-unmodeled-package-parts, load-workbook-with-chart-drawing-does-not-crash, embedded-chart-read-write, image-range-anchor-edit-as-mode-honored.