The product quantity selector has gone through a number of changes. To understand this latest PR, I want to start by summarizing its recent history:

• Prior to 7.2.0, if the product quantity selector's min and max values were the same (ie, it could not be modified), it would render as a hidden input. Otherwise, it rendered as a regular number input.
• In 7.2.0 we merged a change making it so the selector would render even when the min and max values are the same (but in readonly mode). The idea was to make the UI consistent across products, and make the purchase quantity obvious to customers.
• This met with resistance, with many users reporting they preferred the previous behaviour.
• CSS based attempts at a compromise [1] [2] in which we would hide the input if the product was sold individually were attempted, but also proved to be problematic.

So, in this change, we essentially restore the behavior we had before the 7.2.0 release. However, we are also tweaking an existing filter hook and adding one more, making it possible to achieve the objectives of the original PR without implementing a template override.

Updates #36007.

• This PR is a very minor change/addition and does not require testing instructions (if checked you can ignore/remove the next section).

How to test the changes in this Pull Request:

1. Ensure you have one product that is sold individually, and another regular product.
2. Visit the single product page of each:
   • For the sold individually product, the quantity input should be hidden.
   • For the regular product, the quantity input should be visible.
3. Add each item to the cart. Within the cart page:
   • The sold individually product quantity selector should be hidden.
   • The regular product should have a visible quantity selector.

Let's now test that we can achieve the goals of PR#34282 by adding the following code to an appropriate location (such as wp-content/mu-plugins/test-quantity-selectors.php):

<?php
/**
 * This snippet can be used to force the quantity input to display in cases where
 * the input value cannot be changed (which is when the product is set to be sold
 * individually, or when the min and max values are identical).
 */
add_filter( 'woocommerce_quantity_input_args', function ( array $args ) {
	// Only modify inputs where the max value is set and is equal to the min value.
	if ( $args['max_value'] < 1 || $args['min_value'] !== $args['max_value'] ) {
		return $args;
	}

	// Convert into a text input.
	add_filter( 'woocommerce_quantity_input_type', function () {
		return 'text';
	} );

	// Set to readonly.
	$args['readonly'] = true;
	return $args;
} );

Now, if you repeat the above tests, you should find the quantity selector always displays, even for products that are sold individually. However, in these cases it will be a readonly text input.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you created a changelog file for each project being changed, ie pnpm --filter=<project> changelog add?

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.