Impacted plugin
Super Cache
What
Caching set to "all visitors" should not break WP's Theme File Editor loopback test
How
If “Enable caching for all visitors” is selected in Super Cache, PHP files for the currently active theme cannot be edited via the native Wordpress Theme File Editor. This is because the loopback test which confirms the change does not result in a PHP error cannot confirm all is ok, so the change gets reverted. The error “Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.” is shown on screen.
If Super Cache is set to "Disable caching for logged in visitors" then the loopback test is successful and the change to PHP files can be made.
Ideally the loopback test should always be excluded from receiving the cached version, perhaps by user agent or another method you feel is more appropriate
Also see https://wordpress.org/support/topic/breaking-the-wp-theme-file-editor-loopback-test/.
Impacted plugin
Super Cache
What
Caching set to "all visitors" should not break WP's Theme File Editor loopback test
How
If “Enable caching for all visitors” is selected in Super Cache, PHP files for the currently active theme cannot be edited via the native Wordpress Theme File Editor. This is because the loopback test which confirms the change does not result in a PHP error cannot confirm all is ok, so the change gets reverted. The error “Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.” is shown on screen.
If Super Cache is set to "Disable caching for logged in visitors" then the loopback test is successful and the change to PHP files can be made.
Ideally the loopback test should always be excluded from receiving the cached version, perhaps by user agent or another method you feel is more appropriate
Also see https://wordpress.org/support/topic/breaking-the-wp-theme-file-editor-loopback-test/.