- Exposed `HTMLStyleElement` globally in `tests/ui/popoverEngine.test.mjs` to resolve a `ReferenceError` during `instanceof` checks.
- Updated assertion in "applies token-based sizing and arrow positioning" to check the `--popover-arrow-top` CSS variable instead of the `top` property, as JSDOM does not compute layout properties from variables.
- Verified all tests in the file pass.