Show a more meaningful notice when trying to install WP plugin in Git…

… mode. For more information, see https://pantheon.io/docs/start-states/wordpress#20220606-1

wp-content/mu-plugins/pantheon.php

@@ -19,6 +19,9 @@
 	if ( ! defined('RETURN_TO_PANTHEON_BUTTON') || RETURN_TO_PANTHEON_BUTTON ) {
 		require_once 'pantheon/pantheon-login-form-mods.php';
 	}
+	if ( 'dev' === $_ENV['PANTHEON_ENVIRONMENT'] && function_exists( 'wp_is_writable' ) ) {
+		require_once 'pantheon/pantheon-plugin-install-notice.php';
+	}
 	if ( ! defined( 'FS_METHOD' ) ) {
 		/**
 		 * When this constant is not set, WordPress writes and then deletes a 

wp-content/mu-plugins/pantheon/pantheon-plugin-install-notice.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * If a Pantheon site is in Git mode, hide the Plugin installation functionality and show a notice.
+ */
+
+if ( ! wp_is_writable( WP_PLUGIN_DIR ) ) {
+    if ( ! defined( 'DISALLOW_FILE_MODS' ) ) {
+        define( 'DISALLOW_FILE_MODS', true );
+    }
+
+    add_action( 'admin_notices', '_pantheon_plugin_install_notice' );
+    add_action( 'network_admin_notices', '_pantheon_plugin_install_notice' );
+}
+
+function _pantheon_plugin_install_notice() {
+    $screen = get_current_screen(); 
+    // Only show this notice on the plugins page.
+    if ( 'plugins' === $screen->id || 'plugins-network' === $screen->id ) { ?>
+        <div class="update-nag notice notice-warning is-dismissible" style="margin: 5px 6em 15px 0;">
+            <p style="font-size: 14px; margin: 0;">
+                <?php 
+                // Translators: %s is a URL to the user's Pantheon Dashboard.
+                echo wp_kses_post( sprintf( __( 'If you wish to update or add plugins using the WordPress UI, switch your site to SFTP mode from <a href="%s">your Pantheon dashboard</a>.', 'pantheon-systems' ), 'https://dashboard.pantheon.io/sites/' . $_ENV['PANTHEON_SITE'] ) ); 
+                ?>
+            </p>
+        </div>
+        <?php
+    }
+}