diff --git a/src/php/snippet-ops.php b/src/php/snippet-ops.php
index 8f6dc441b..d43f5f056 100644
--- a/src/php/snippet-ops.php
+++ b/src/php/snippet-ops.php
@@ -383,7 +383,7 @@ function update_shared_network_snippets( array $snippets ): bool {
 	}
 
 	$existing_shared_ids = get_site_option( 'shared_network_snippets', [] );
-	$updated_shared_ids = array_values( array_diff( array_merge( $existing_shared_ids, $shared_ids ), $unshared_ids ) );
+	$updated_shared_ids = array_values( array_unique( array_diff( array_merge( $existing_shared_ids, $shared_ids ), $unshared_ids ) ) );
 
 	if ( $existing_shared_ids === $updated_shared_ids ) {
 		return false;
@@ -393,21 +393,30 @@ function update_shared_network_snippets( array $snippets ): bool {
 
 	// Deactivate the snippet on all sites if necessary.
 	if ( $unshared_ids ) {
-		$sites = get_sites( [ 'fields' => 'ids' ] );
+		// Every site, not the first hundred that get_sites() returns by default:
+		// one left out would keep running a snippet that is no longer shared.
+		$sites = get_sites(
+			[
+				'fields' => 'ids',
+				'number' => 0,
+			]
+		);
 
 		foreach ( $sites as $site ) {
 			switch_to_blog( $site );
 			$active_shared_ids = get_option( 'active_shared_network_snippets' );
 
 			if ( is_array( $active_shared_ids ) ) {
-				$active_shared_ids = array_diff( $active_shared_ids, $unshared_ids );
+				$active_shared_ids = array_values( array_diff( $active_shared_ids, $unshared_ids ) );
 				update_option( 'active_shared_network_snippets', $active_shared_ids );
 			}
 
 			clean_active_snippets_cache( code_snippets()->db->ms_table );
-		}
 
-		restore_current_blog();
+			// Each switch is undone before the next, as one restore only steps
+			// back a single switch and would leave the request on another site.
+			restore_current_blog();
+		}
 	}
 
 	return true;
@@ -807,6 +816,7 @@ function save_snippet( $snippet ): ?Snippet {
 
 		$snippet->id = $wpdb->insert_id;
 		$updated = get_snippet( $snippet->id, $snippet->network );
+		$updated->shared_network = $snippet->shared_network;
 		$updated->code_error = $snippet->code_error;
 		$updated->code_error_trace = $snippet->code_error_trace;
 		do_action( 'code_snippets/create_snippet', $updated, $table );
@@ -828,6 +838,7 @@ function save_snippet( $snippet ): ?Snippet {
 		clean_snippets_cache( $table );
 
 		$updated = get_snippet( $snippet->id, $snippet->network );
+		$updated->shared_network = $snippet->shared_network;
 		$updated->code_error = $snippet->code_error;
 		$updated->code_error_trace = $snippet->code_error_trace;
 
diff --git a/tests/unit/Snippets/Shared_Network_Snippets_Test.php b/tests/unit/Snippets/Shared_Network_Snippets_Test.php
new file mode 100644
index 000000000..e35f80c7b
--- /dev/null
+++ b/tests/unit/Snippets/Shared_Network_Snippets_Test.php
@@ -0,0 +1,244 @@
+<?php
+
+namespace Code_Snippets;
+
+use Code_Snippets\Model\Snippet;
+
+/**
+ * Tests for sharing network snippets with subsites.
+ *
+ * Whether a network snippet is shared is not a column on the snippet. It is
+ * held as a list of IDs in a network option, and each subsite keeps its own
+ * list of the shared snippets it has switched on.
+ *
+ * @group snippet-ops
+ * @group multisite
+ */
+class Shared_Network_Snippets_Test extends UnitTestCase {
+
+	/**
+	 * Set up before each test.
+	 *
+	 * @return void
+	 */
+	public function set_up() {
+		parent::set_up();
+
+		if ( ! is_multisite() ) {
+			$this->markTestSkipped( 'Sharing with subsites only exists on multisite installs.' );
+		}
+
+		delete_site_option( 'shared_network_snippets' );
+	}
+
+	/**
+	 * Build an unsaved network snippet.
+	 *
+	 * @param bool $shared Whether it is shared with subsites.
+	 *
+	 * @return Snippet
+	 */
+	private function network_snippet( bool $shared ): Snippet {
+		return new Snippet(
+			[
+				'name'           => 'Shared network fixture',
+				'code'           => 'body { color: red; }',
+				'scope'          => 'site-css',
+				'network'        => true,
+				'shared_network' => $shared,
+			]
+		);
+	}
+
+	/**
+	 * Save an existing network snippet with a different share setting, as the editor does.
+	 *
+	 * @param int  $id     Snippet identifier.
+	 * @param bool $shared Requested share setting.
+	 *
+	 * @return Snippet|null
+	 */
+	private function save_with_sharing( int $id, bool $shared ): ?Snippet {
+		$edit = clone get_snippet( $id, true );
+		$edit->shared_network = $shared;
+
+		return save_snippet( $edit );
+	}
+
+	/**
+	 * IDs currently shared with subsites.
+	 *
+	 * @return array
+	 */
+	private function shared_ids(): array {
+		return (array) get_site_option( 'shared_network_snippets', [] );
+	}
+
+	/**
+	 * A new snippet saved as shared is shared.
+	 *
+	 * @return void
+	 */
+	public function test_new_snippet_saved_as_shared_is_shared(): void {
+		$saved = save_snippet( $this->network_snippet( true ) );
+
+		$this->assertSame( [ $saved->id ], $this->shared_ids() );
+		$this->assertTrue( $saved->shared_network, 'the saved snippet should report the setting it was saved with' );
+	}
+
+	/**
+	 * A new snippet saved without sharing is not shared.
+	 *
+	 * @return void
+	 */
+	public function test_new_snippet_saved_without_sharing_is_not_shared(): void {
+		$saved = save_snippet( $this->network_snippet( false ) );
+
+		$this->assertSame( [], $this->shared_ids() );
+		$this->assertFalse( $saved->shared_network );
+	}
+
+	/**
+	 * Sharing can be switched on for a snippet that already exists.
+	 *
+	 * @return void
+	 */
+	public function test_sharing_can_be_switched_on_for_an_existing_snippet(): void {
+		$id = save_snippet( $this->network_snippet( false ) )->id;
+
+		$saved = $this->save_with_sharing( $id, true );
+
+		$this->assertSame( [ $id ], $this->shared_ids() );
+		$this->assertTrue( $saved->shared_network );
+		$this->assertTrue( get_snippet( $id, true )->shared_network, 'the setting should still be on when the snippet is read again' );
+	}
+
+	/**
+	 * Sharing can be switched off again.
+	 *
+	 * @return void
+	 */
+	public function test_sharing_can_be_switched_off(): void {
+		$id = save_snippet( $this->network_snippet( true ) )->id;
+
+		$saved = $this->save_with_sharing( $id, false );
+
+		$this->assertSame( [], $this->shared_ids() );
+		$this->assertFalse( $saved->shared_network );
+	}
+
+	/**
+	 * A save that does not mention sharing leaves it as it was.
+	 *
+	 * @return void
+	 */
+	public function test_saving_without_a_share_setting_keeps_the_current_one(): void {
+		$id = save_snippet( $this->network_snippet( true ) )->id;
+
+		$edit = new Snippet(
+			[
+				'id'      => $id,
+				'name'    => 'Shared network fixture',
+				'code'    => 'body { color: blue; }',
+				'scope'   => 'site-css',
+				'network' => true,
+			]
+		);
+		save_snippet( $edit );
+
+		$this->assertSame( [ $id ], $this->shared_ids() );
+	}
+
+	/**
+	 * Hooks on a save are handed the share setting that was saved.
+	 *
+	 * @return void
+	 */
+	public function test_save_hooks_receive_the_requested_share_setting(): void {
+		$seen = [];
+		$record = function ( Snippet $snippet ) use ( &$seen ) {
+			$seen[] = $snippet->shared_network;
+		};
+
+		add_action( 'code_snippets/create_snippet', $record );
+		add_action( 'code_snippets/update_snippet', $record );
+
+		$id = save_snippet( $this->network_snippet( true ) )->id;
+		$this->save_with_sharing( $id, false );
+
+		remove_action( 'code_snippets/create_snippet', $record );
+		remove_action( 'code_snippets/update_snippet', $record );
+
+		$this->assertSame( [ true, false ], $seen );
+	}
+
+	/**
+	 * Saving a shared snippet repeatedly lists its ID once.
+	 *
+	 * @return void
+	 */
+	public function test_repeated_saves_list_a_shared_snippet_once(): void {
+		$id = save_snippet( $this->network_snippet( true ) )->id;
+
+		$this->save_with_sharing( $id, true );
+		$this->save_with_sharing( $id, true );
+
+		$this->assertSame( [ $id ], $this->shared_ids() );
+	}
+
+	/**
+	 * A list that already holds repeats is tidied by the next save.
+	 *
+	 * @return void
+	 */
+	public function test_existing_repeats_are_removed_on_the_next_save(): void {
+		$id = save_snippet( $this->network_snippet( true ) )->id;
+		update_site_option( 'shared_network_snippets', [ $id, $id, $id ] );
+
+		$this->save_with_sharing( $id, true );
+
+		$this->assertSame( [ $id ], $this->shared_ids() );
+	}
+
+	/**
+	 * Switching sharing off switches the snippet off on every subsite using it.
+	 *
+	 * @return void
+	 */
+	public function test_switching_sharing_off_deactivates_the_snippet_on_every_subsite(): void {
+		$id = save_snippet( $this->network_snippet( true ) )->id;
+		$other_id = $id + 1000;
+		$subsites = [ $this->factory()->blog->create(), $this->factory()->blog->create() ];
+
+		foreach ( $subsites as $subsite ) {
+			update_blog_option( $subsite, 'active_shared_network_snippets', [ $id, $other_id ] );
+		}
+
+		$this->save_with_sharing( $id, false );
+
+		foreach ( $subsites as $subsite ) {
+			$this->assertSame(
+				[ $other_id ],
+				array_values( (array) get_blog_option( $subsite, 'active_shared_network_snippets' ) ),
+				'the snippet should no longer be active on a subsite, and others it had on should be untouched'
+			);
+		}
+	}
+
+	/**
+	 * Switching sharing off leaves the request on the site it started on.
+	 *
+	 * @return void
+	 */
+	public function test_switching_sharing_off_returns_to_the_original_site(): void {
+		$id = save_snippet( $this->network_snippet( true ) )->id;
+		$this->factory()->blog->create();
+		$this->factory()->blog->create();
+		$original = get_current_blog_id();
+
+		$this->save_with_sharing( $id, false );
+
+		$this->assertSame( $original, get_current_blog_id(), 'the save should finish on the site it started on' );
+		$this->assertFalse( ms_is_switched(), 'no switch to another site should be left open' );
+	}
+}