Customize: Client-side notification error is unexpectedly cleared when no corresponding server-side validation

[westonruter]

Setting validation can be added both by adding a validate_callback (or sanitize_callback) on the server, or it can be added by adding an error Notification via JS (see also ​Make/Core post):

wp.customize( 'blogdescription', function( setting ) {
        var errorCode = 'capital_P_dangit';
        setting.bind( function( value ) {
                var notification;
                if ( /[Ww]ordpress/.test( value ) ) {
                        notification = new wp.customize.Notification(
                                errorCode,
                                {
                                        type: 'error',
                                        message: 'Capital P Dangit!'
                                }
                        );
                        setting.notifications.add( errorCode, notification );
                } else {
                        setting.notifications.remove( errorCode );
                }
        } );
} );

However, if the server lacks corresponding validation (which normally it should) the setting is currently marked as valid on the server and the error gets unexpectedly cleared on the client.

[westonruter]

In order for returning an error/notification to work properly, api._handleSettingValidities needs to only remove notification errors that are no longer valid which are fromServer. See 39770.0.diff​.

[swissspidy]

@westonruter Is this ready for a commit?

[westonruter]

In 40319:

Customize: Prevent client-side validation from being cleared when no corresponding server-side validation is present.

See #36944.
Fixes #39770.

[westonruter]

Re-opening for consideration in 4.7.4.

[swissspidy]

In 40345:

Customize: Prevent client-side validation from being cleared when no corresponding server-side validation is present.

See #36944.
Fixes #39770.

Merges [40319] to the 4.7 branch.