* Edits to active plugins which cause PHP fatal errors will no longer auto-deactivate the plugin. Supersedes #39766.
* Introduce sandboxed PHP file edits for active themes, preventing accidental whitescreening of a user's site when introducing a fatal error.
* After writing a change to a PHP file for an active theme or plugin, perform loopback requests on the file editor admin screens and the homepage to check for fatal errors. If a fatal error is encountered, roll back the edited file and display the error to the user to fix and try again.
* Introduce a secure way to scrape PHP fatal errors from a site via `wp_start_scraping_edited_file_errors()` and `wp_finalize_scraping_edited_file_errors()`.
* Moves file modifications from `theme-editor.php` and `plugin-editor.php` to common `wp_edit_theme_plugin_file()` function.
* Refactor themes and plugin editors to submit file changes via Ajax instead of doing full page refreshes when JS is available.
* Use `get` method for theme/plugin dropdowns.
* Improve styling of plugin editors, including width of plugin/theme dropdowns.
* Improve notices API for theme/plugin editor JS component.
* Strip common base directory from plugin file list. See #24048.
* Factor out functions to list editable file types in `wp_get_theme_file_editable_extensions()` and `wp_get_plugin_file_editable_extensions()`.
* Scroll to line in editor that has linting error when attempting to save. See #41886.
* Add checkbox to dismiss lint errors to proceed with saving. See #41887.
* Only style the Update File button as disabled instead of actually disabling it for accessibility reasons.
* Ensure that value from CodeMirror is used instead of `textarea` when CodeMirror is present.
* Add "Are you sure?" check when leaving editor when there are unsaved changes.
Supersedes [41560].
See #39766, #24048, #41886.
Props westonruter, Clorith, melchoyce, johnbillion, jjj, jdgrimes, azaozz.
Fixes#21622, #41887.
Built from https://develop.svn.wordpress.org/trunk@41721
git-svn-id: http://core.svn.wordpress.org/trunk@41555 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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