Unable to Navigate Autocomplete Suggestions via Keyboard in Theme & Plugin File Editor

This issue affects the built-in Theme and Plugin File Editor within the WordPress admin area.

When editing a file inside the editor, keyword suggestions appear as we type. However, there is a problem specifically with .php files:

• When typing in a .php file, the autocomplete suggestion list appears.
• Attempting to navigate the suggestions using the up/down arrow keys does not work as expected.
• The selection remains stuck on the first suggested item, making it impossible to move through the list via keyboard input.
• However, when selecting suggestions using the mouse cursor, they work as expected.

Here’s a screen recording demonstrating the issue : (​https://www.awesomescreenshot.com/video/37998611?key=262d499496bfcc778da2e6975eaaf248)

This issue is only present in .php files. Other file types such as .css, .js, and .html work correctly and allow navigation via keyboard input.

Steps to Reproduce:

1. Open the WordPress Theme or Plugin File Editor.
2. Edit a .php file and start typing a function or keyword that triggers the autocomplete suggestion list.
3. Try navigating the suggestion list using the up/down arrow keys.
4. Notice that the selection remains stuck on the first item and does not move.
5. Now, try selecting a suggestion with the mouse cursor—it works fine.

Expected Behavior:

• The up/down arrow keys should allow smooth navigation through the autocomplete suggestions, just like they do for .css, .js, and .html files.

Actual Behavior:

• Keyboard navigation is broken for .php files, preventing users from selecting suggestions efficiently.

Environment Details:

• WordPress Version: 6.7.2
• Browser: Chrome, Firefox
• Theme & Plugins: Tested with default theme Twenty Twenty-One and no plugin active.

Would appreciate further insights or confirmation from the Core team on this. Thank you!