Using a custom query var on a static front page results in wrong $post

[mechter]

Steps to reproduce:

• Create a page and make it the static front page
• Add a new query var via filter in theme's functions.php: add_filter( 'query_vars', function( $vars ) { array_push( $vars, 'buggy' ); return $vars; } );
• Open the front page setting the newly added query var (e.g. "​https://example.com/?buggy=yep")

Result: displays the latest blog post
Expected result: displays front page

[joyously]

I was able to reproduce this using a child theme with TwentyFifteen as the parent. TwentyFifteen does not have a front-page.php file.

[bhargavbhandari90]

I am also replicating this.
Replying to joyously:

I was able to reproduce this using a child theme with TwentyFifteen as the parent. TwentyFifteen does not have a front-page.php file. Also on TwentySixteen, which does not have a front-page.php either.

[SirLouen]

Reproduction Report

Description

✅ This report validates that the issue can be reproduced.

Environment

• WordPress: 6.9-alpha-60093-src
• PHP: 8.2.29
• Server: nginx/1.29.1
• Database: mysqli (Server: 8.4.6 / Client: mysqlnd 8.2.29)
• Browser: Chrome 139.0.0.0
• OS: Windows 10/11
• Theme: Twenty Twenty-Five 1.3
• MU Plugins: None activated
• Plugins:
  • BBB Testing Dolly
  • Test Reports 1.2.0

Instructions

1. The ones provided by OP.

Actual Results

1. ✅ Error condition occurs (reproduced).

Additional Notes

• There is something weird around this line

​https://github.com/wordpress/wordpress-develop/blob/9ca38ce47b7a8a9c9e916e6aff39ec772ccbba55/src/wp-includes/class-wp-query.php#L1057
That array_diff is not correctly formulated, as the result unexpectedly is this buggy query var. I'm assuming that the idea is, that if any of those query vars existed, not to show the front page, but the corresponding search page. A patch is needed.

Trac ticket: https://core.trac.wordpress.org/ticket/40521