* Invalidate PHP opcache after file is updated to ensure `include` will ''include'' the written changes.
* Define `WP_ADMIN` when activating plugin in sandbox so plugin code targeting admin will be loaded.
* Do actions that get triggered when loading the admin to ensure plugin code runs that could cause errors on plugin editor screen (and lock out access).
* Fix ability to re-activate a plugin after editing a PHP file other than the main plugin file, and ensure PHP fatal error will be displayed in such cases.
* Consolidate duplicated code into `plugin_sandbox_scrape()` and re-use in `activate_plugin()`.
* Show an error notice instead of a success notice when a file is updated but a plugin was deactivated due to a fatal error.
* Update style of warning when editing an active plugin to be styled as an actual warning notice.
See #12423, #21622.
Fixes#39766.
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