implemented with phpstan/phpstan-src#1878

If you see the message, PHPStan is restarted so the message is correct. If you want this to behave better, you need to improve this code: https://github.com/composer/xdebug-handler/blob/ced299686f41dce890debac69273b47ffe98a40c/src/XdebugHandler.php#L625-L667

S1 - using phpstan with the option --xdebug option on a env w/ xdebug disabled

php vendor/bin/phpstan analyse --xdebug src
! [NOTE] You are running with "--xdebug" enabled, but the Xdebug PHP extension is not active. The process will not halt
! at breakpoints.

phpstan runs fast, expected

S2 - using phpstan without the --xdebug option on a env w/ xdebug enabled

php vendor/bin/phpstan analyse src
! [NOTE] The Xdebug PHP extension is active, but "--xdebug" is not used. This may slow down performance and the process
! will not halt at breakpoints.

phpstan runs fast, unexpected

S3 - using phpstan with the --xdebug option on a env w/ xdebug enabled

php vendor/bin/phpstan analyse --xdebug src

no message
phpstan runs slow, unexpected

S4 - workaround, instead of using the phpstan --xdebug option, use the php -d xdebug option, on a env w/ xdebug enabled

php -d xdebug.mode=off vendor/bin/phpstan analyse src

no message
phpstan runs fast, expected

perhaps just remove the --xdebug option and update documentation to use the php -d xdebug option

S2 - using phpstan without the --xdebug option on a env w/ xdebug enabled

This is no longer true. No message is printed since phpstan/phpstan-src@53d0d21.

S3 - using phpstan with the --xdebug option on a env w/ xdebug enabled

This is no longer true. A message is printed since phpstan/phpstan-src@1d4ede1