Declare Document.scrollingElement as HTMLElement | null

[Copilot]

Document.scrollingElement was declared Element | null while documentElement and body — the only two values it can return per CSSOM View — are both declared HTMLElement.

// Previously TS2322; now assignable.
const element: HTMLElement | null = document.scrollingElement;

Changes

• lib.dom.d.ts: readonly scrollingElement: HTMLElement | null;
• New compiler test documentScrollingElementType.ts covering assignability to HTMLElement | null.
• Baseline update for mappedTypeRecursiveInference.types, which structurally expands Document.

Note

lib.dom.d.ts is generated; a corresponding change in TypeScript-DOM-lib-generator is needed so this survives the next regeneration.

• Fixes Inconsistent types between Document scrollingElement and documentElement #60838