This is a recent change  to the spec for parsing the windowFeatures argument to window.open(). Previous to this change, window.open(url,'','popup=true') interpreted the 'popup=true' to mean popup is false. With this change, 'true' counts as a truthy value.  https://github.com/whatwg/html/pull/7425
Having `popup=true` result in popup being false is counterintuitive and confusing. Making this change should make boolean features easier to use and understand.
Final published standard: Recommendation, Living Standard, Candidate Recommendation, or similar final form
Status in Chromium
Enabled by default
Consensus & Standardization
Search tagswindow.open, windowFeatures,
Last updated on 2022-05-14