TYPO3 Core

Prepending some option !== preselecting exisiting option by value

just sayin ...

This is ONLY a documentation task.
The "defaultValue" fallback is done within:

\TYPO3\CMS\Fluid\ViewHelpers\Form\AbstractFormFieldViewHelper::getPropertyValue()

    ObjectAccess::getPropertyPath($formObject, $this->arguments['property']);

$formObject = TYPO3\CMS\Form\Domain\Runtime\FormRuntime which implements \ArrayAccess.

If $formObject[$formElementIdentifier] has no value (and "value" means "submitted data"), then the FormRuntime returns the value from the property "defaultValue".
If you set the viewhelpers "value" property with the elements "defaultValue" property than this value will be selected every time!
Please make a formDefinition and add 2 pages

type: Form
identifier: select-test
label: select-test
prototypeName: standard
renderables:
  -
    type: Page
    identifier: page-1
    label: Step
    renderables:
      -
        type: MultiSelect
        identifier: multiselect-1
        label: 'Multi select'
        properties:
          options:
            foo1: bar1
            foo2: bar2
            foo3: bar3
        type: MultiSelect
        identifier: multiselect-1
        label: 'Multi select'
        defaultValue:
          - foo1
          - foo3
  -
    type: SummaryPage
    identifier: summarypage-1
    label: 'Summary step'

select "bar2" and go to the summary step. Now go back to the first step. As you can see, patchset 1 will select the selection from the "defaultValue" property again.
But without the patchset 1 every is still fine.

Or use it by code (without the patchset 1):

formDefinition:

type: Form
identifier: select-test
label: select-test
prototypeName: standard
renderables:
  -
    type: Page
    identifier: page-1
    label: Step
    renderables:
      -
        type: MultiSelect
        identifier: multiselect-1
        label: 'Multi select'
        type: MultiSelect
        identifier: multiselect-1
        label: 'Multi select'
  -
    type: SummaryPage
    identifier: summarypage-1
    label: 'Summary step'

ext_localconf.php

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['ext/form']['initializeFormElement'][1506431770]
    = \TRITUM\FormExample\Hooks\SelectHook::class;

Classes/Hooks/SelectHook.php

<?php
namespace TRITUM\FormExample\Hooks;
use TYPO3\CMS\Form\Domain\Model\Renderable\RenderableInterface;

class SelectHook
{
    public function initializeFormElement(RenderableInterface $renderable)
    {
        if ($renderable->getUniqueIdentifier() === 'select-test-1-multiselect-1') {
            $renderable->setProperty('options', [
                'foo1' => 'bar1',
                'foo2' => 'bar2',
                'foo3' => 'bar3'
            ]);
            $renderable->setDefaultValue([
                'foo1',
                'foo3'
            ]);
        }
    }
}

Works for me. Since the "defaultValue" property is missing within the docs, this should be covered by this patch, but NOT the template changes.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56991

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/56991