Validation icon on form tab

[brynwhyman]

Overview

There's currently an open enhancement to add better handling to validation messages outside of a form tab that a CMS user has in focus, see: Validation messages for hidden tabs

Some early investigation into that enhancement has found that there's probably some regressed behaviour at play.

This issue aims to look at a more immediate fix utilising some of the code that's appeared to regress, with the addition of the icon indicators outlined in Validation messages for hidden tabs.

Acceptance criteria

• When a validation error happens in a child tab (i.e. Content > 'Custom-tab') I can clearly identify the tab that it is contained in
• Focus is retained on the form in view when the save was attempted and I am not redirected to the validation error

Notes

• This has been raised separately from Validation messages for hidden tabs silverstripe-cms#2081 assuming that it will be a quick fix and possible for inclusion in a patch release.

• @sachajudd to raise a PR to add the new icon to the font set.

• This will not address adding icon indicators in the root tabs, i.e 'Content', 'Settings', etc

Designs

Patch designs
CMS Design System - Validation errors future state

PRs

• NEW Show icons on tabs with invalid form field values #1111

[brynwhyman]

Found this issue while looking at this: #1110

[Cheddam]

I tested the related PR (#1111) using the DataObject::validate() method, and found that it would render errors on fields, but not top-level errors / messages, and the tab icons did not render.

Sample code

class HomePage extends BaseHomePage
{
    private static $db = [
        'ExampleField' => 'Varchar',
    ];
    
    public function getCMSFields()
    {
        $fields = parent::getCMSFields();
        
        $fields->addFieldToTab('Root.Test', TextField::create('ExampleField', 'Example Field'));
        
        return $fields;
    }
    
    public function validate()
    {
        $validation = parent::validate();
        
        // Just return a validation error every time
        $validation->addFieldError('ExampleField', 'This field cannot exist.');

        // Never renders :(
        $validation->addError('This page cannot exist.');
        
        return $validation;
    }
}