const PAGE = ` `; add_task(async function() { const url = "data:text/html," + encodeURI(PAGE); await BrowserTestUtils.withNewTab( { gBrowser, url, }, async function(browser) { let popup = await openSelectPopup("click"); let arrowSB = popup.shadowRoot.querySelector(".menupopup-arrowscrollbox"); is( arrowSB.getBoundingClientRect().width, 600, "Should be the right size" ); } ); });