Requested

Fixes #2288

Implemented

• Observe elements with { box: "border-box" } when supported.
• Prefer ResizeObserverEntry.borderBoxSize for reported dimensions.
• Preserve fallback behavior for older browsers by falling back to contentRect and to plain observe(target) when observe options are unsupported.
• Map inlineSize/blockSize for vertical and sideways writing modes.
• Added unit coverage for border-box sizing, vertical writing mode mapping, sideways writing mode mapping, contentRect fallback, unsupported observe options fallback, and non-TypeError observe failures.

Not covered

No changes to window resize handling

Validated

• git diff --check
• Attempted targeted validation with:
• yarn jest packages/shared/src/dom-events/resize/resizeElement.test.ts --runInBand
• yarn prettier --check packages/shared/src/dom-events/resize/resizeElement.ts packages/shared/src/dom-events/resize/resizeElement.test.ts

Both Yarn commands could not run in this checkout because the Yarn node_modules state file is missing and dependencies are not currently installed.