Follow-up to #3012, where this inconsistency surfaced.

The gap

FormView.columns is a spec key — view.zod declares it right next to splitDirection / splitSize. But only ObjectForm's simple path (ObjectForm.tsx:914) and ModalForm actually read it. TabbedForm and SplitForm derived the grid width from the per-section columns alone:

const formColumns = declaredCols.length ? Math.max(...declaredCols) : 1;

So a form view declaring columns: 3 rendered 3 columns in a modal and single-column as a tab or a split — one piece of metadata laying out differently depending on which host picked it up. ObjectForm was already spreading columns through to both; it was simply dropped on arrival, so nothing downstream could tell the key had been authored.

Per AGENTS.md #0 this is a compliance bug, not a preference: the renderer was silently discarding a declared spec key.

The fix

Both hosts now resolve the grid the way the others already did:

explicit form `columns`  ??  widest section's `columns`  ??  1

The precedence reflects that the two keys answer different questions — the view's columns is how wide the grid is; a section's columns is how densely that section fills it (via per-field colSpan). columns is declared on TabbedFormSchema / SplitFormSchema accordingly.

Tests

Three cases added to packages/plugin-form/src/sectionColumns.test.tsx (the existing home for grouped-section layout), verified failing first — both host tests failed expected null not to be null on the 3-column grid class, while the section-fallback case passed, confirming the baseline was precisely "view columns ignored, section columns fine":

• TabbedForm honours the view's columns
• SplitForm honours the view's columns
• falls back to the widest section when the view declares none (and does not invent a third column)

38 files / 293 tests green across plugin-form + the form renderer; turbo type-check 12/12; lint 0 errors.

Deliberately out of scope

WizardForm is the remaining host that ignores view-level columns (WizardForm.tsx:531 passes only currentSection.columns || 1). Left out on purpose: a wizard's grid is per-step, and sectionFormLayout(fields, cols) currently collapses width and density into one number — honouring the view key there means introducing the width/density split properly (applyAutoColSpan(body, formColumns, section.columns) + containerGridColsFor), which also changes the default width of every existing wizard step. That deserves its own PR rather than being smuggled into a precedence swap. Happy to do it next.

No browser verification for this one — the change is a resolved integer feeding existing, already-verified grid classes, and the three DOM assertions cover it.

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