Fix shadowRoot instanceof check in unsupported browsers

Editor assessment

Compact and clear. The subject pins the fragile construct — a shadowRoot instanceof check — and the failure condition in browsers lacking Shadow DOM support, so the log reads well. The runtime-dom scope is correct and the PR reference documents provenance. The repeated 'fix' after the type is redundant; dropping the verb or rephrasing would be cleaner style. There is no body, but for a one-line compatibility guard the subject is nearly sufficient, though naming an affected browser would have helped testers. At about 78 characters the header modestly exceeds the 72-character guideline and may truncate in GitHub's interface. Patch-level fix classification is unambiguous, and changelog tooling will handle it without friction.