It works as expected when adding one separator but when adding multiple via xml

<menu type="separator">--</menu>

it creates only one (the last one) in the database. The problem relies on the alias creation by using the "current-date" - the alias has to be unique. I've quickly tested my theory by changing the following:

$data['alias']        = (string) $child;

to:

$data['alias']        = ((string) $child->attributes()->alias) ?: (string) $child;

and setting a custom alias on my separators like so:

<menu type="separator" alias="one">--</menu>
<menu type="separator" alias="two">--</menu>
...

Now the database elements got created. So the alias generation does not take care of the "uniqueness" of created / added elements here and replace the before created when adding the next separator to the database.

What you can do in your XML to bring it to work is to give them unique content:

<menu type="separator">one</menu>
<menu type="separator">two</menu>
...

The Problem is that in unpatched state the items got then also rendered like a default menu item including its label. So installing a component that makes use of this feature will lead to rendered menu items "one" "two"...

So how about generating an unique alias here if the type is separator?
An option would be to count the separator types and increase a counter... someting clean and understandable like "component-separator-n":

...
$separatorCounter = 0;
foreach ($this->getManifest()->administration->submenu->menu as $child) {
...
if((string) $child->attributes()->type === 'separator') {
                $data['alias']        = $option . '-separator-' . $separatorCounter++;
            }
...

This would result in:
com-componentname-separator-0, com-componentname-separator-1, ...

@marcorensch Thanks for testing and the feedback, I hadn't thought about testing it for multiple separators.

I like your first suggestion of updating just the alias assignment line,
$data['alias'] = ((string) $child->attributes()->alias) ?: (string) $child;
as the better solution and is simple to implement and document.

Using this example in the manifest .xml is shows the various combinations.

        <submenu>
            <menu link="option=com_testing">Menu Item 1</menu>
            <menu type="separator">-----------------------</menu>
            <menu link="option=com_testing">Menu Item 2</menu>
            <menu type="separator" alias="second">.....................</menu>
            <menu link="option=com_testing">Menu Item 3</menu>
            <menu type="separator" alias="third">______________________</menu>
        </submenu> 

Specifying an alias for the first separator is optional and only required for subsequent separators.

For Joomla instances that don't have this PR then only one separator will be saved in the table but the characters in the title can be used to create a character based separator.

This snippet of code in ..\administrator\modules\mod_menu\src\Menu\CssMenu.php (#461) is used to empty the title property and leave it to the css to draw the separator. For instance prior to this PR the condition is never met therefore the characters in the title are displayed.

            if ($item->type === 'separator' && $itemParams->get('text_separator') == 0) {
                $item->title = '';
            }

@Irata I would say that sounds like a good solution to cover all eventualities.

I have updated the the extra line to allow the alias to be specified when creating multiple separators.

Please review and test.

@Irata and @marcorensch thanks for the good work - @marcorensch could please mark a successful test in the issue tracker and remember: we still need another successful test

I have updated the Testing instructions with a sample manifest .xml that can be used to test this PR.

I have tested this item ✅ successfully on e05f694

I have tested this item ✅ successfully on e05f694

RTC