Forms: Fix source filtering in classic view for responses management (#42641)

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/14031937053

Upstream-Ref: Automattic/jetpack@fdb3704

Author: ntsekouras

CHANGELOG.md

@@ -15,6 +15,7 @@ This is an alpha version! The changes listed here are not final.
 
 ### Fixed
 - Components: Add __next40pxDefaultSize to controls, preventing deprecation notices.
+- Forms: Fix `source` filtering in classic view for responses management
 - Forms: Prevent custom label font sizes preventing animated label font size reduction
 - Placeholder should always display if it's a non-empty string
 

src/contact-form/class-admin.php

@@ -862,8 +862,13 @@ public function grunion_source_filter_results( $query ) {
 			return;
 		}
 
-		// Don't apply to the filter dropdown query
-		if ( $query->query_vars['fields'] === 'id=>parent' ) {
+		/**
+		 * In the wp-admin list we perform two queries that trigger the `pre_get_posts` hook.
+		 * One is for the main list and the other is for the `source` dropdown filter.
+		 * We need to explicitly check one unique parameter between the two queries to avoid
+		 * filtering the dropdown query. The dropdown query is in `get_all_parent_post_ids`.
+		 */
+		if ( $query->query_vars['posts_per_page'] === 100000 ) {
 			return;
 		}