FIX Focus on first tab with field that failed validation

[emteknetnz]

Update: we're unlikely to merge this, because even though it's a regression from previous behaviour, we'll probably enhance this and put alert icons on tabs with invalid form fields, instead of tab switching

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Partial fix for silverstripe/silverstripe-cms#2081

Based on Robbie noticing this silverstripe/silverstripe-cms#2081 (comment)

Seems like this is a regression of functionality that used to work. At some point we must have change the classes used for tabs and validation messages, as well as possibly how tabs are structured. Likely while going from SS3 to SS4.

Testing setup:

Page.php

<?php

namespace {

    use SilverStripe\CMS\Model\SiteTree;
    use SilverStripe\Forms\RequiredFields;
    use SilverStripe\Forms\TextField;

    class Page extends SiteTree
    {
        private static $db = [
            'ABC' => 'Varchar',
            'DEF' => 'Varchar',
            'XYZ' => 'Varchar',
            'S22' => 'Varchar',
        ];

        private static $has_one = [];

        public function getCMSFields()
        {
            $fields = parent::getCMSFields();
            $fields->addFieldToTab("Root.Second", TextField::create("ABC"));

            // used to test javascript scroll into view
            $fields->addFieldToTab("Root.Third",TextField::create("null1"));
            $fields->addFieldToTab("Root.Third",TextField::create("null2"));
            $fields->addFieldToTab("Root.Third",TextField::create("null3"));
            $fields->addFieldToTab("Root.Third",TextField::create("null4"));
            $fields->addFieldToTab("Root.Third",TextField::create("null5"));
            $fields->addFieldToTab("Root.Third",TextField::create("null6"));
            $fields->addFieldToTab("Root.Third",TextField::create("null7"));
            $fields->addFieldToTab("Root.Third",TextField::create("null8"));
            $fields->addFieldToTab("Root.Third",TextField::create("null9"));
            $fields->addFieldToTab("Root.Third",TextField::create("null10"));
            $fields->addFieldToTab("Root.Third",TextField::create("null11"));
            $fields->addFieldToTab("Root.Third",TextField::create("null12"));
            $fields->addFieldToTab("Root.Third",TextField::create("null13"));
            $fields->addFieldToTab("Root.Third",TextField::create("null14"));

            $fields->addFieldToTab("Root.Third", TextField::create("DEF"));
            
            $fields->addFieldToTab("Root.Fourth", TextField::create("XYZ"));
            return $fields;
        }

        public function getSettingsFields()
        {
            $fields = parent::getSettingsFields();
            $fields->addFieldToTab("Root.Settings", TextField::create('S22'));
            return $fields;
        }

        public function getCMSValidator() {
            return new RequiredFields(array('DEF', 'XYZ', 'S22'));
        }
    }
}

1. Click Save without filling in any fields
2. It should focus on the Tab "Third" and you should see a "DEF" is required validation message.
3. Fill the DEF field in, click save
4. You should now be focused on the Tab "Fourth" and see a "XYZ" is required validation message.

[emteknetnz]

I'm not sure how we'd go about solve the issue originally in issue, which was related to validating errors in different top level tabs ("Content" | "Settings" | "History") - in the example code about there is new field in settings S22 which can be 'left blank' while in the "Content" tab, and the Page will still save correctly. Only when in the "Settings" tab and clicking "Save" will validation get triggered for S22

What this fix does do is fix this for the next tab level down validation issues - there was an issue on an internal project with tabs at the lower level under the top level "Content" tab: i.e. "Root.Main" | "Root.Second"

[emteknetnz]

Behat failure looks to be an existing failure on admin 1.6