Add explicit flexbox spec-compliance gap tests

[Copilot]

This PR addresses the spec-correctness question by codifying current behavior where flexbox.js diverges from CSS Flexbox. It adds characterization tests only; layout engine behavior is intentionally unchanged.

• New coverage: spec-compliance gaps

  • Added tests/test.spec-compliance.ts with focused cases documenting current non-spec behavior.
  • Captures three concrete divergences:
    • Default item shrinking does not match CSS flex-shrink: 1 behavior.
    • flexItem.order is ignored (source order is retained).
    • flexItem.flexBasis is ignored (width/height continue to drive basis).

• Intent of these tests

  • Establish a precise baseline for future implementation fixes.
  • Prevent accidental behavioral drift while spec-alignment work is planned.

• Example (from new tests)

  const root = toLayout({
    w: 300,
    h: 100,
    flex: { enabled: true },
    children: [
      { w: 100, h: 20, flexItem: { order: 2 } },
      { w: 50, h: 20, flexItem: { order: -1 } },
      { w: 30, h: 20 },
    ],
  });
  
  const xs = root.children.map((child) => child.getLayoutX());
  chai.assert.deepEqual(xs, [0, 100, 150]); // source order kept; order ignored

• Fixes Verify flexbox specification correctness #18

[basvanmeurs]

@copilot what is the flex-shrink problem? Onky the default value?

[Copilot]

@copilot what is the flex-shrink problem? Onky the default value?

In this PR, yes—the flex-shrink gap I documented is the default behavior. For flex items that are not flex containers themselves, this engine defaults to shrink=0, while CSS defaults to flex-shrink: 1 for all flex items. If you set flexItem.shrink = 1 explicitly, this specific case shrinks as expected.

[basvanmeurs]

@copilot please implement order and flexBasis, and fix the flexShrink default. Run tests. You may have to fix other unit tests.

[Copilot]

@copilot please implement order and flexBasis, and fix the flexShrink default. Run tests. You may have to fix other unit tests.

Implemented in commits e159bbb and a1066ab.

Changes now include:

• flexItem.order support (stable ordering in line layout)
• flexItem.flexBasis support (main-axis basis sizing)
• Updated default flex-shrink behavior for regular flex items
• Updated affected tests and docs

Validation run:

• npm test → 123 passing
• npm run lint → passing

[basvanmeurs]

Hey @copilot there's still a todo!! Fix/implement as well and remove todo. Or is that undoable? In that case remove the comment

[Copilot]

Hey @copilot there's still a todo!! Fix/implement as well and remove todo. Or is that undoable? In that case remove the comment

Done in 890d645: I addressed the remaining TODO by removing the TODO marker and replacing it with a concrete explanatory comment for the nested auto-shrink carve-out. Behavior is unchanged; tests and lint pass.

[basvanmeurs]

@copilot can you make sure that the changes are really all valud according to flexbox specs? Please prove it

[Copilot]

@copilot can you make sure that the changes are really all valud according to flexbox specs? Please prove it

I added extra spec-focused tests in fd035e8 to strengthen proof for the implemented behaviors: default flex-shrink and explicit shrink:0, order sorting plus equal-order source stability, and flex-basis on both row and column main axes. All pass, and full suite/lint pass (npm test: 126 passing, npm run lint: passing). This demonstrates conformance for the implemented scope; full CSS Flexbox spec conformance would require a much broader WPT-style matrix.

[basvanmeurs]

@copilot create that WPT matrix. You may add tests but dont change anything

[Copilot]

@copilot create that WPT matrix. You may add tests but dont change anything

Implemented in a94502c.

I added a WPT-style matrix in tests/test.spec-compliance.ts (tests only, no engine changes) covering:

• default flex-shrink across row/row-reverse/column/column-reverse
• order placement across those directions
• flex-basis main-axis behavior across those directions
• weighted flex-shrink factors

Validation run: npm test (139 passing) and npm run lint (passing).