[gethinwebster]

Description

Moves the sticky header logic that Table and Container use into one shared internal module, src/internal/components/sticky-header/. This lets the table components (TableRoot and friends) reuse it for their sticky header in a follow-up PR, instead of having a second copy.

What moves, with no change to public APIs:

• From Table: the code that tucks the real header up under its sticky copy and scrolls focused rows clear of it (use-sticky-header-copy.ts, sticky-scrolling.ts).
• From Table's column widths: watching each header cell for size changes (use-cells-resize-observer.ts).
• From Container: the sticky offset calculation and the stuck state (use-sticky-position.ts, use-header-stuck.ts).
• The styles that hide the sticky copy's scrollbar, now a shared mixin.

Table, Container and Cards now call the shared code and are otherwise unchanged. One small difference: Table used to re-collect the header cells it watches only when the visible columns changed. It now re-collects them after each render, so it also picks up cells that remount for other reasons.

Related links, issue #, if available: n/a

How has this been tested?

• Existing Container, Table and Cards unit tests pass without changes. This includes the column-width test that covers header cells resizing with their content.
• New unit tests for the shared cell resize observer. One of them covers a cell remounting between renders, and it fails against the old behaviour.
• New unit tests for the shared sticky header copy hook's scrollToTop and scrollToRow, which had no coverage in the existing Table either.
• The sticky offset tests now render the shared useStickyPosition hook and check the top it applies, so they also cover the scrolling-parent and app layout <main> checks.
• Window resizing was profiled against main: React renders, commits and per-component re-render counts per resize are identical.
• Documenter and test-utils snapshots are unchanged.
• Compiled CSS for Table, Container and Cards matches main once class-name hashes are normalised. The only difference is where the hidden-scrollbar rule sits.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.82%. Comparing base (cc35fe2) to head (9d1b40a).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5212      +/-   ##
==========================================
+ Coverage   97.75%   97.82%   +0.06%     
==========================================
  Files        1003     1006       +3     
  Lines       32534    32602      +68     
  Branches    12082    12111      +29     
==========================================
+ Hits        31805    31893      +88     
+ Misses        684      667      -17     
+ Partials       45       42       -3     

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[Copilot]

🟢 Approval recommended

No active review issues remain for the shared sticky-header extraction.

No open issues 🎉

What changed in this PR

Extracts sticky-header internals into a shared module for Table, Container, and Cards, preparing for reuse by atomic table components without changing public APIs.

Changes:

• Centralizes sticky positioning, stuck-state tracking, header synchronization, scrolling helpers, and scrollbar styles.
• Extracts cell resize observation and refreshes observed cells after each render to handle remounts.
• Adds resize-observer unit tests and relocates sticky-offset tests.

File	Description
src/​table/​use-sticky-header.ts	Delegates to shared header synchronization.
src/​table/​use-column-widths.tsx	Uses the shared cell resize observer.
src/​table/​styles.scss	Uses the shared hidden-scrollbar mixin.
src/​internal/​components/​sticky-header/​use-sticky-position.ts	Houses sticky positioning and offset calculation.
src/​internal/​components/​sticky-header/​use-sticky-header-sync.ts	Houses header overlap and focus-scrolling logic.
src/​internal/​components/​sticky-header/​use-header-stuck.ts	Extracts stuck-state tracking.
src/​internal/​components/​sticky-header/​use-cells-resize-observer.ts	Tracks cell replacements and resize notifications.
src/​internal/​components/​sticky-header/​sticky-scrolling.ts	Relocates scrolling helpers and removes stale parameter documentation.
src/​internal/​components/​sticky-header/​mixins.scss	Defines shared hidden-scrollbar styles.
src/​internal/​components/​sticky-header/​__tests__/​use-cells-resize-observer.test.tsx	Tests resizing, replacement cells, callback updates, and cleanup.
src/​internal/​components/​sticky-header/​__tests__/​sticky-offset.test.ts	Relocates offset tests and updates imports.
src/​container/​use-sticky-header.ts	Delegates to shared sticky positioning.
src/​cards/​index.tsx	Imports scrolling helpers from the shared module.

_{Review effort: Balanced}

[jperals]

Nit: why is this function exported? It doesn't seem to be used anywhere else

[gethinwebster]

good catch, it was only used in tests. I've un-exported it and updated tests

[jperals]

This part of the refactor does not look very necessary. useStickyHeader now only calls useStickyHeaderSync, and useStickyHeaderSync does not seem to be called anywhere else.

[gethinwebster]

updated by renaming this and importing it directly rather than having this file that just imports and re-exports it. this hook will also be imported in the atomic table components when we implement sticky header there (that's what this PR is unblocking)

[Copilot]

🟢 Approval recommended

The shared sticky-header refactor has no active review issues blocking approval.

No open issues 🎉

_{Review effort: Balanced}