useDebouncedMemo never re-arms mountedRef: accessibility tree never renders under React StrictMode

[shinji-incorta]

Summary

Under React StrictMode (i.e. any development build wrapped in <StrictMode>), the
accessibility tree never renders — document.querySelectorAll('[role="gridcell"]').length
stays 0 for the lifetime of the grid. Screen readers get an unlabelled <canvas>, and
tooling that reads the a11y tree (e2e tests, audits) sees nothing. Production builds without
StrictMode are unaffected.

Cause

useDebouncedMemo in dist/esm/common/utils.js (6.0.3):

export function useDebouncedMemo(factory, deps, time) {
    const [state, setState] = React.useState(factory);
    const mountedRef = React.useRef(true);
    React.useEffect(() => () => {
        mountedRef.current = false;          // ← only ever set to false
    }, []);
    const debouncedSetState = React.useRef(debounce(x => {
        if (mountedRef.current) {            // ← gate
            setState(x);
        }
    }, time));
    React.useLayoutEffect(() => {
        if (mountedRef.current) {            // ← gate
            debouncedSetState.current(() => factory());
        }
    }, deps);
    return state;
}

StrictMode mounts, unmounts and remounts each component in development. The cleanup sets
mountedRef.current = false; the remount never sets it back, because the ref is only
initialised on first render. Both guards are then permanently false, so no dependency change
ever produces a new value and the memo keeps whatever factory() returned at first render.

The only consumer is accessibilityTree in internal/data-grid/data-grid.js, whose factory
begins:

if (width < 50 || experimental?.disableAccessibilityTree === true) return null;

At first render the grid is unmeasured (width === 0), so that first value is null — and it
is never replaced.

Evidence

Instrumented 6.0.3 with React 19.2 in Chromium, logging inside the memo and inside
onVisibleRegionChangedImpl:

condition	values seen by the memo	[role=gridcell]
StrictMode on (as shipped)	width: 0, accessibilityHeight: 1 on every run	0
StrictMode on, DataEditor given numeric width/height	still width: 0	0
StrictMode on, onVisibleRegionChanged logged	clientWidth: 1078, clientHeight: 395 — the measurement does arrive and setClientSize runs	0
StrictMode removed, nothing else changed	width: 1022 → 842, accessibilityHeight: 12	36

The third row is the decisive one: width genuinely goes 0 → 1078 via clientSize, and the
memo still never re-runs, even though width is the first entry in its dependency array.

Reproduction

Render any <DataEditor> inside <StrictMode> and, once cells are painted, evaluate
document.querySelectorAll('[role="gridcell"]').length → 0. Remove <StrictMode> and the
same page returns one node per visible cell, with the expected accessible text.

Suggested fix

Re-arm the ref on mount:

React.useEffect(() => {
    mountedRef.current = true;
    return () => {
        mountedRef.current = false;
    };
}, []);

Alternatively drop the mountedRef guard entirely — since React 18, calling a state setter
after unmount is a no-op rather than a warning, so the guard no longer earns its keep.

Happy to open a PR if that would help. We are carrying the four-line change as a local patch
in the meantime.

[islittlezhou]

you know, this project is dead!