feat(ui): header slot + SidebarLayout.DefaultHeader for the top bar

[interacsean]

Summary

Adds an official, composable extension point for the app-shell top bar. SidebarLayout gains a full-region header?: ReactNode slot (mirroring the existing sidebar slot), defaulting to the built-in SidebarLayout.DefaultHeader.

Note: this PR was reworked following review (discussion). It originally added a narrow headerActions prop; it now uses a full-region header slot + building blocks, for consistency with sidebar/children and to avoid a proliferation of one-off header props.

API

Three tiers of customization:

// 1. Default — built-in header
<SidebarLayout />

// 2. Extend the built-in header (common case: add a notification bell, user menu…)
<SidebarLayout
  header={
    <SidebarLayout.DefaultHeader
      actions={[
        <NotificationBell key="bell" />,
        // `actions` REPLACES the right-hand cluster, so include the switcher to keep it
        <AppearanceSwitcher key="appearance" />,
      ]}
    />
  }
/>

// 3. Replace the header entirely
<SidebarLayout header={<MyCustomHeader />} />

SidebarLayout.DefaultHeader

The built-in top bar: sidebar trigger + breadcrumb on the left, an actions cluster on the right.

• actions — ReactNode | ReactNode[] (optional). The entire right-hand cluster, in an opinionated horizontal, vertically-centered row.
  • Defaults to [<AppearanceSwitcher />], so the zero-config header is unchanged.
  • ⚠️ Passing actions replaces the whole cluster, including the appearance switcher — include <AppearanceSwitcher /> explicitly to keep it (AppearanceSwitcher is a public export). actions={[]} renders an empty right side.
  • This keeps the API a single slot rather than accumulating one-off props (hideAppearanceSwitcher, headerLeft, …).

Namespacing

• DefaultHeader is exposed as SidebarLayout.DefaultHeader and as a top-level DefaultHeader export.
• DefaultSidebar is now also exposed as SidebarLayout.DefaultSidebar (top-level DefaultSidebar export retained for backwards compatibility).

Why

Resolves tailor-inc/platform-planning#484 — Customizable NavBar (header). Previously there was no supported way to extend the top bar, so consumers (e.g. TRM) queried the header DOM, created a portal target with a MutationObserver, and createPortal-ed their controls in — fragile and tied to internal structure. This replaces that with a first-class slot.

Changes

• packages/core/src/components/sidebar/default-header.tsx — new DefaultHeader (trigger + breadcrumb + actions)
• packages/core/src/components/sidebar/sidebar-layout.tsx — header slot; namespace SidebarLayout.DefaultHeader / .DefaultSidebar
• packages/core/src/components/sidebar/index.ts, packages/core/src/index.ts — exports
• packages/core/src/components/sidebar/sidebar-layout.test.tsx — 13 tests (header slot, actions replace/default/empty, namespace)
• Docs: docs/components/{sidebar-layout,default-header (new),default-sidebar,appearance-switcher}.md, catalogue/src/fundamental/components.md + regenerated skill reference
• examples/vite-app/src/App.tsx — demonstrates the new API
• .changeset/ — minor bump

Follow-ups (out of scope)

• Publicly export lower-level header primitives (breadcrumb, sidebar trigger) so tier-3 custom headers can reuse them.
• Migrate other internal consumers to the namespaced SidebarLayout.DefaultSidebar.

Testing

• oxlint — 0 warnings/errors · tsc --noEmit — clean · vitest — 1206 passed
• Emitted .d.ts carries namespace SidebarLayout { DefaultSidebar; DefaultHeader }; example app type-checks as a real consumer
• Browser-verified: actions render next to / in place of the switcher; switcher dropdown works when composed into actions

🤖 Generated with Claude Code

[IzumiSy]

@interacsean

WIP API thought: this may already be in flux, but sharing the comment anyway in case it’s useful.

My understanding of SidebarLayout’s customization API so far was that it is mostly organized around replacing whole regions, such as sidebar and children. From that perspective, headerActions feels a bit different, since it introduces a partial customization point inside the built-in header rather than replacing the header as a whole.

One reason I’m slightly hesitant about headerActions specifically is that once we add a narrowly scoped prop like this, it tends to invite more props in the same direction over time — e.g. headerLeft, headerRight, headerContent, hideAppearanceSwitcher, renderBreadcrumbs, etc. At that point the built-in header starts becoming incrementally configurable through many special-case props, which makes the API feel less coherent.

If we want to open up the header as an extension point, I wonder if something like header?: ReactNode would be more consistent with the existing API shape.

More generally, one thing I’ve been trying to optimize for in this kind of API is to keep extension points like header broadly open as ReactNode, and then provide the pieces that go into that slot as building blocks at different levels. That tends to balance flexibility and convention well: the slot itself stays simple and general, while consumers can choose between full replacement, default helpers like SidebarLayout.DefaultHeader, or lower-level building blocks depending on how much customization they need.

Then, for cases where consumers only want to slightly extend the built-in header, we could provide a helper like SidebarLayout.DefaultHeader, following the same idea as DefaultSidebar: the main component exposes a full-region customization point, while a default implementation remains available as a convenience for partial customization. Since AppearanceSwitcher is already public, this would still make it easy to reconstruct the default setup when needed, without adding a one-off prop like headerActions.

Related to that, thinking about symmetry, I’m also starting to feel that DefaultSidebar itself wants to live under the SidebarLayout namespace as well. In practice it is rarely useful outside the context of SidebarLayout, so SidebarLayout.DefaultSidebar / SidebarLayout.DefaultHeader feels like a more cohesive API shape. We could still keep the top-level DefaultSidebar export for compatibility, but the namespaced form seems like a nicer direction for discoverability and consistency.

[interacsean]

Thanks @IzumiSy — this steer makes sense, and I've reworked the PR to follow it. Dropped the one-off headerActions prop in favour of a full-region slot + building blocks:

SidebarLayout gains a header?: ReactNode slot (mirrors sidebar), defaulting to the built-in SidebarLayout.DefaultHeader. Three tiers of customization:

// 1. default
<SidebarLayout />

// 2. extend the built-in header (common case)
<SidebarLayout
  header={
    <SidebarLayout.DefaultHeader
      actions={[<NotificationBell key="bell" />, <AppearanceSwitcher key="appearance" />]}
    />
  }
/>

// 3. replace entirely
<SidebarLayout header={<MyCustomHeader />} />

SidebarLayout.DefaultHeader (also a top-level DefaultHeader export) renders trigger + breadcrumb on the left and an actions cluster on the right. Key decision, following your "switcher is a building block" point:

• actions defaults to [<AppearanceSwitcher />] — so the zero-config header is unchanged.
• actions replaces the entire right-hand cluster, including the switcher. If you customize and still want it, you include <AppearanceSwitcher /> in the array. This deliberately avoids a hideAppearanceSwitcher-style boolean and keeps it a single slot. (AppearanceSwitcher is already a public export.)

Symmetry: DefaultSidebar is now also exposed as SidebarLayout.DefaultSidebar; the top-level export stays for back-compat, matching your suggestion.

Docs updated across docs/components (sidebar-layout, new default-header, default-sidebar, appearance-switcher) + the catalogue/skill reference, and the example app demonstrates it.

Two things I intentionally scoped out of this PR as follow-ups, lmk if you'd rather fold them in:

1. Publicly exporting the lower-level header primitives (breadcrumb, sidebar trigger) so tier-3 full-replacement headers can reuse them — right now a full custom header would rebuild those.
2. Migrating other internal consumers to the namespaced SidebarLayout.DefaultSidebar.

Does this shape match what you had in mind?