plugin.php

<?php
if ( ! isset( $_GET['test-remove-theme-supports'] ) )
	return;

// Use ?test-remove-theme-supports=1 or 2 or 3
switch ( $_GET['test-remove-theme-supports'] ) {
	case 1:
		define( 'HEADER_IMAGE_WIDTH', 123 );
		add_theme_support( 'custom-header' );

		if ( 123 != get_theme_support( 'custom-header', 'width' ) )
			die( 'something went wrong, was not able to set width.' );

		if ( defined( 'HEADER_IMAGE_HEIGHT' ) )
			die( 'something went wrong, why was height defined?' );

		// just-in-time
		do_action( 'wp_loaded' );

		if ( ! defined( 'HEADER_IMAGE_HEIGHT' ) )
			die( 'something went wrong, height should be defined by now.' );

		break;
	case 2:
		add_theme_support( 'custom-header', array( 'width' => 456 ) );

		if ( 456 != get_theme_support( 'custom-header', 'width' ) )
			die( 'was not able to set width to 456.' );

		define( 'HEADER_IMAGE_WIDTH', 123 );
		add_theme_support( 'custom-header', array( 'width' => 789 ) );

		if ( 456 != get_theme_support( 'custom-header', 'width' ) )
			die( 'was not able to keep width at 456.' );

		// just-in-time
		do_action( 'wp_loaded' );

		if ( 456 != get_theme_support( 'custom-header', 'width' ) )
			die( 'was not able to set keep width at 456.' );

		if ( ! defined( 'HEADER_IMAGE_HEIGHT' ) )
			die( 'something went wrong, height should be defined by now.' );

		break;
	case 3:
		define( 'HEADER_IMAGE_WIDTH', 123 );
		add_theme_support( 'custom-header' );

		define( 'HEADER_IMAGE_HEIGHT', 456 );
		add_theme_support( 'custom-header' );

		if ( 123 != get_theme_support( 'custom-header', 'width' ) )
			die( 'Could not set width to 123' );

		if ( 456 != get_theme_support( 'custom-header', 'height' ) )
			die( 'Could not set height to 456' );

		break;
}

die( 'everything looks ok' );