Customiser fails to honour theme support defaults for background images

[philipjohn]

When using add_theme_support() to set defaults for the background image, an unexpected consequence of using the customiser is that those defaults are ignored.

For example, consider the following code, used in functions.php;

add_theme_support( 'custom-background', array(
	'default-image' => get_stylesheet_directory_uri() . '/bg.png',
	'default-repeat' => 'repeat-x',
	'default-position-x' => 'center',
	'default-attachment' => 'scroll',
) );

As would be expected, the customiser honours the "default-image" setting but the repeat, position-x and attachment settings are all ignored. Instead, the customiser chooses "repeat", "left" and "fixed" respectively.

This means that the first Save in the customiser will overwrite the theme's default settings, changing the look and feel of the site despite the user not having changed any settings.

[philipjohn]

Proposed fix

[danielbachhuber]

Couple of things:

• global $_wp_theme_features; should be dropped
• default-attachment defaults to scroll in the $_wp_theme_features, but the default in Customizer was fixed. I suspect it's fine to change it to scroll, but wanted to flag just in case.

[philipjohn]

Removing redundant global

[ocean90]

In 29103:

Customizer: Use theme support defaults for background images.

props philipjohn.
fixes #28586.