The web upgrade process is meant to progressively display messages to the user, by flushing the output. This is meant to be implemented by WebInstaller_Upgrade::startLiveBox() and the WebInstallerOutput::flush() call in WebInstallerOutput::addHTML().

My observations indicate that support for this is broken -- upgrading does not display any progress output. Presumably this is because MW_NO_OUTPUT_BUFFER is not defined in mw-config/index.php.

Version: 1.22.0
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=51576