fix: sync sticky head on body resize via ResizeObserver

[melikhov-dev]

Problem

When a DataTable is rendered with stickyHead: MOVING and cell content changes asynchronously (e.g. via useEffect + dangerouslySetInnerHTML), the sticky header column widths become desynchronised from the body.

Root cause: syncHeadWidths() was only triggered by window.resize. An async re-render of <tbody> children doesn't fire TableHead.componentDidUpdate (head props/state didn't change) and doesn't dispatch a resize event, so the sticky head stays at the stale widths from the initial render.

The symptom persists until something causes a window.resize — e.g. toggling DevTools or calling window.dispatchEvent(new Event('resize')) manually.

Fix

Subscribe a ResizeObserver to _box in componentDidMount whenever stickyHead is enabled. When the observer fires, a requestAnimationFrame debounce coalesces rapid bursts into a single syncHeadWidths() call. Both the observer and any pending rAF are cleaned up in componentWillUnmount.

// componentDidMount
if (stickyHead && this._box && typeof ResizeObserver !== 'undefined') {
    this._bodyResizeObserver = new ResizeObserver(() => {
        if (this._syncHeadRaf) return;
        this._syncHeadRaf = requestAnimationFrame(() => {
            this._syncHeadRaf = 0;
            this.syncHeadWidths();
        });
    });
    this._bodyResizeObserver.observe(this._box);
}

// componentWillUnmount
this._bodyResizeObserver?.disconnect();
if (this._syncHeadRaf) cancelAnimationFrame(this._syncHeadRaf);

Compatibility: guarded by typeof ResizeObserver !== 'undefined' — no-op in SSR and older browsers.

Tests

Adds the first component-level test suite (src/test/DataTable.stickyHead.test.tsx, 8 tests):

• ResizeObserver is created and observes .data-table__box when stickyHead is enabled
• ResizeObserver is not created when stickyHead is disabled
• disconnect() is called on unmount (no leak)
• A resize event schedules exactly one rAF
• Multiple rapid resize events coalesce into one rAF
• The guard resets correctly after each rAF flush
• No throw when ResizeObserver is unavailable

Also adds required test infrastructure: jest-environment-jsdom, @testing-library/react, @types/jest, SCSS mock, and updates jest.config.js to pick up .test.tsx files.

[gravity-ui-bot]

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