Fix/rotation recalc compound transforms

[geoffyoungs]

PR description

Fix rotation centre recalculation corrupting compound transforms

When changing attributes on rotated elements, the rotation center recalculation had two bugs:

1. It fired for all attribute changes (stroke-width, fill, opacity, etc.), not just geometry-affecting ones (x, y, width, height, etc.). Non-geometric changes don't move the bbox centre, so the recalculation was unnecessary and destructive.
2. The new rotation center was computed through all remaining transforms in the list, including pre-rotation ones like translate. This caused the translate to leak into the centre, producing incorrect rotation.

Both bugs existed in undo.js (changeSelectedAttributeNoUndoMethod) and history.js (ChangeElementCommand.apply/unapply). The history.js version was worse — it replaced the entire transform attribute with just rotate(...), destroying any other transforms.

To replicate the bug:

Use this sample SVG:

This branch is deployed to: https://geoffyoungs.github.io/svgedit/
Compare with the upstream demo at https://svgedit.netlify.app/editor/index.html

To test the bug:

1. Open both editors
2. Import the pinwheel SVG (or draw shapes with compound transforms)
3. Select the group and change stroke-width
4. On the upstream demo, the shapes jump — on the fixed version, they stay in place

Checklist

Note that we require UI tests to ensure that the added feature will not be
nixed by some future fix and that there is at least some test-as-documentation
to indicate how the fix or enhancement is expected to behave.

• - Added Cypress UI tests
• - Ran npm test, ensuring linting passes and that Cypress UI tests keep
  coverage to at least the same percent (reflected in the coverage badge
  that should be updated after the tests run)
• - Added any user documentation. Though not required, this can be a big
  help both for future users and for the PR reviewer.

NB: Claude Code was used to identify and fix this issue.

Summary by Sourcery

Fix rotation center recalculation so that transforms on rotated elements are preserved and compound transforms remain stable when attributes change or history operations are applied.

Bug Fixes:

• Restrict rotation center recalculation to geometry-affecting attributes and compute the new center using only post-rotation transforms, preventing corrupt rotation on elements with compound transforms.
• Preserve existing transform lists when updating rotation centers during undo/redo instead of replacing the entire transform with a simple rotate(), avoiding loss of translate/scale transforms.

Tests:

• Add visual fixtures and a Playwright-based screenshot script to document and manually verify the rotation recalculation behaviour on elements with compound transforms.

[sourcery-ai]

Reviewer's Guide

Refactors rotation-center recalculation so it only runs for geometry-affecting attribute changes and correctly preserves compound transform lists, and adds visual/demo assets and supporting test scaffolding for the bug.

Sequence diagram for attribute change with safe rotation-center recalculation

sequenceDiagram
  actor User
  participant EditorUI
  participant SvgCanvas
  participant ChangeElementCommand
  participant relocateRotationCenter
  participant SVGElement as SvgElement

  User->>EditorUI: Change attribute (e.g. stroke-width, x, width)
  EditorUI->>SvgCanvas: changeSelectedAttributeNoUndoMethod(attr, newValue, elems)
  SvgCanvas->>SvgElement: Update attribute
  SvgCanvas->>SvgElement: getRotationAngle(elem)
  alt angle is zero or attr is not bbox-affecting
    SvgCanvas-->>EditorUI: No rotation recalc
  else angle nonzero and attr bbox-affecting
    SvgCanvas->>SvgElement: getTransformList(elem)
    SvgCanvas->>SvgElement: Remove existing rotate transform
    SvgCanvas->>SvgElement: utilsGetBBox(elem)
    SvgCanvas->>SvgElement: transformListToTransform(tlist, n, end)
    SvgCanvas->>SvgElement: transformPoint(bboxCenter, centerMatrix)
    SvgCanvas->>SvgElement: createSVGTransform().setRotate(angle, center.x, center.y)
    SvgCanvas->>SvgElement: insertItemBefore(newRotate, n)
  end

  User->>EditorUI: Perform undo or redo
  EditorUI->>ChangeElementCommand: apply() or unapply()
  ChangeElementCommand->>SvgElement: Apply stored attribute values
  ChangeElementCommand->>relocateRotationCenter: relocateRotationCenter(elem, changedAttrNames)
  relocateRotationCenter->>SvgElement: getRotationAngle(elem)
  alt no bbox-affecting attrs or angle is zero
    relocateRotationCenter-->>ChangeElementCommand: Return without changes
  else
    relocateRotationCenter->>SvgElement: getTransformList(elem)
    relocateRotationCenter->>SvgElement: Remove rotate transform at index n
    relocateRotationCenter->>SvgElement: getBBox(elem)
    relocateRotationCenter->>SvgElement: transformListToTransform(tlist, n, end)
    relocateRotationCenter->>SvgElement: transformPoint(bboxCenter, centerMatrix)
    relocateRotationCenter->>SvgElement: createSVGTransform().setRotate(angle, center.x, center.y)
    relocateRotationCenter->>SvgElement: insertItemBefore(newRotate, n)
  end

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Class diagram for ChangeElementCommand and rotation recalc helper

classDiagram
  class ChangeElementCommand {
    - elem : Element
    - oldValues : Object
    - newValues : Object
    + constructor(elem, attrs, values)
    + apply()
    + unapply()
  }

  class relocateRotationCenterHelper {
    + BBOX_AFFECTING_ATTRS : Set~string~
    + relocateRotationCenter(elem, changedAttrs) void
  }

  class SvgUtilities {
    + getRotationAngle(elem) number
    + getBBox(elem) DOMRect
  }

  class SvgMath {
    + getTransformList(elem) SVGTransformList
    + transformListToTransform(tlist, fromIndex, toIndex) SVGTransform
    + transformPoint(x, y, matrix) SVGPoint
  }

  ChangeElementCommand --> relocateRotationCenterHelper : uses
  relocateRotationCenterHelper --> SvgUtilities : uses
  relocateRotationCenterHelper --> SvgMath : uses

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File-Level Changes

Change	Details	Files
Add shared rotation-center relocation helper that updates only the rotate transform while preserving the rest of the transform list and using only post-rotation transforms to compute the new center.	Introduce BBOX_AFFECTING_ATTRS set and relocateRotationCenter(elem, changedAttrs) in history.js using math.js helpers (getTransformList, transformListToTransform, transformPoint, getBBox). In ChangeElementCommand.apply and .unapply, replace manual bbox/rotate recomputation and full transform overwrite with relocateRotationCenter using newValues/oldValues attribute keys. Ensure rotation center calculation transforms the bbox center only through transforms that follow the removed rotation in the list, falling back to identity if none remain.	packages/svgcanvas/core/history.js
Restrict rotation-center recomputation on live attribute changes to bbox-affecting attributes and fix center calculation to ignore pre-rotation transforms.	Define local BBOX_AFFECTING_ATTRS in changeSelectedAttributeNoUndoMethod and gate rotation-center updates on angle != 0, attr !== 'transform', and attr being bbox-affecting. After removing the existing rotate transform, compute the bbox via utilsGetBBox and build a center transform matrix only from transforms that come after the rotation, defaulting to identity when none exist. Use transformPoint with the new center matrix to derive the updated rotation center and reinsert a rotate(angle, cx, cy) transform at the original index.	packages/svgcanvas/core/undo.js
Add visual documentation and tooling to demonstrate and capture the rotation-recalculation bug and its fix.	Create rotation-recalc-demo.html visual page explaining the two rotation bugs (history.js full-transform replacement and undo.js pre-rotation leakage) with annotated SVG examples. Add a Playwright-based screenshot.mjs script to capture a PNG screenshot of the rotation-recalc demo HTML for documentation/CI use. Introduce additional visual and unit test artifacts (compound-transform-bug.svg and rotation-recalc.test.js stub) as anchors for future automated tests of this behavior.	tests/visual/rotation-recalc-demo.html tests/visual/screenshot.mjs tests/visual/compound-transform-bug.svg tests/unit/rotation-recalc.test.js

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[sourcery-ai]

Hey - I've left some high level feedback:

• The BBOX_AFFECTING_ATTRS set is now defined twice (in history.js and inside changeSelectedAttributeNoUndoMethod); consider centralizing this in a shared module/constant so the attribute list stays consistent and isn’t duplicated.
• In relocateRotationCenter, tlist.removeItem(n) is called before getBBox(elem) and an early return on a falsy bbox would leave the rotation permanently removed; it would be safer to compute/validate the bbox before mutating the transform list or otherwise ensure the rotation is restored on failure paths.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `BBOX_AFFECTING_ATTRS` set is now defined twice (in `history.js` and inside `changeSelectedAttributeNoUndoMethod`); consider centralizing this in a shared module/constant so the attribute list stays consistent and isn’t duplicated.
- In `relocateRotationCenter`, `tlist.removeItem(n)` is called before `getBBox(elem)` and an early `return` on a falsy bbox would leave the rotation permanently removed; it would be safer to compute/validate the bbox before mutating the transform list or otherwise ensure the rotation is restored on failure paths.

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[jfhenon]

plz fix the linter errors so I can merge your (very useful) PR

[geoffyoungs]

Apologies - I should have checked that before submitting. SVG-Edit is an excellent and mature tool - I was both surprised and excited to find a legitimate bug!