Fix active tab underline highlight in Appearance Add Theme view

[CookieDarb]

What?

Closes #447

Fixes the bug where the colored underline bar/highlight always stays stuck on "Popular" in the Appearance -> Add Theme (theme installer) screen and does not update when other tabs ("Latest", "Block Themes", "Favorites") are clicked.

Why?

The theme installer (theme-install.php) runs in a chromeless iframe context (desktop_mode_chromeless=1). WordPress Core's legacy Backbone router updates the URL in place using history.replaceState or pushState, but it doesn't correctly update the DOM's active state tab classes in this environment because the initial browse query parameter from window.location.search was only evaluated once on initial page load, ignoring subsequent client-side URL changes.

How?

• includes/themes-tabs.php:
  • Refactored desktop_mode_theme_install_active_tab_script() inline JavaScript to dynamically evaluate the browse URL query parameter on every DOM check via a helper function getBrowseParam(), rather than caching the initial query parameter.
  • Updated the MutationObserver on .filter-links to fetch the fresh browse parameter on every mutation, correctly updating the current class and aria-current="page" attribute dynamically.
• tests/phpunit/tests/desktopModeInjectAppearanceTabs.php:
  • Added test_theme_install_active_tab_script_registered_and_emits_dynamic_browse_param() to assert that the active tab script is hooked to admin_footer at priority 100 and outputs the inline JS helper.

Testing Instructions

1. Open the Desktop environment and go to Appearance -> Add Theme (the "Popular" tab is active by default).
2. Click on the "Latest", "Block Themes", or "Favorites" tabs.
3. Verify that the colored highlight bar/underline correctly moves to the active tab.

Screencast

Screen.Recording.2026-07-29.at.4.57.38.PM.mov

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity
Model(s): Gemini
Used for: Reviewing the existing implementation and suggesting the changes. Final decisions and edits were made by me.

[AllTerrainDeveloper]

This looks great!

Thank you!