Ah! Someone gave me the answer! Chrome recently applied a new default of "lax" to the "SameSite" parameter of cookies. Lax was not lax enough; I needed to set SameSite=none. https://www.mediawiki.org/wiki/Manual:$wgCookieSameSite $wgCookieSameSite='none'; Thanks!!
Hello all! I have a wiki (Mediawiki v1.35.1) that works when unframed – that is, when not embedded as an iframe. When iframed, the wiki login form: …works with: Firefox, 85.0 Linux Firefox, 86.0 Windows …and does NOT work with: Chrome, 89.0 Windows “Not working” means authentication is blocked and this error is displayed: There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking. Please resubmit the form."