you have to implement preprocess() in your router, where assign correct Itemid,
see discussion #35621

@Irata Please check the previous comment, and if it solves your issue let us know and close the issue here, or we can close it for you. Thanks in advance.

Is this the closest thing to up to date information about the router in Joomla 4? I see a few other issues in the past year or so pointing people to this documentation, is it still current?

@Fedik, I don't have my own router I am using what I believe to be the default one, Joomla\CMS\Router\Route.

The current behavior is the expected behavior. There are serious issues with defaulting to the current Itemid for any given URL. It is one of the big reasons for "duplicate content" in Joomla among other things.

Closing as expected behaviour. Feel free to re-open if you think this is wrong.

Please re-open. The duplicate content issue is bs. At a minimum this expected behaviour must be documented somewhere other than in one persons head.

Re-opening due to previous comment.

It is not is expected behaviour to me as the backward compatibility page for J4 makes no mention to the extent of changes you have to make to an existing component to overcome the removal of this default behaviour that a lot of existing code relies on. Any documentation on the 'modern' router seems to be updated J3.x articles but still no mention that $Itemid was no longer going to be handled and there is noting other than a few comments I have seen that says MenuRules class will now handle it.

I want to move sites to Joomla 4 so I can start developing new code with Joomla 4 therefore I have a choice of either spending days or weeks reworking a lot code to make it work under both Joomla 4 and Joomla 3 just to get around this issue, sorry expected behaviour, or I put these few lines of code into SiteRouter.php so I can move sites to Joomla 4 and deal with the older code as more useful documentation becomes availabletime and budget allows.

` if (!$itemid)
{
$item = $this->menu->getItem($this->getVar('Itemid'));

        if ($item !== null && $item->component === $uri->getVar('option'))
        {
            $uri->setVar('Itemid', $item->id);
        }
    }

`

I don't mind whether you close this issue or leave it open, however I have unfortunately spent too many days already getting to this point to waste any more time on it, I know enough now to proceed to Joomla 4 and hopefully this issue will assist others that find their way here looking for an answer why $Itemid is missing in Joomla 4.

I don't have my own router

How do you build your URLs then?

It is not is expected behaviour to me as the backward compatibility page for J4 makes no mention to the extent of changes you have to make to an existing component to overcome the removal of this default behaviour that a lot of existing code relies on.

It is expected for J4.
You have to set Itemid in preprocess() in your router. (Or use new router rule Joomla\CMS\Component\Router\Rules\MenuRules), look my previous comment. It not a "big change".

@Fedik My apologies, I do have a router.php that extends Joomla\CMS\Component\Router\RouterBase with a build and parse method that in their comments says they only come into play when SEF is on, which in this case it is not.

I now understand where your suggested preprocess method gets placed. I will report back when I have had a chance to play with it.

Thank you.

their comments says they only come into play when SEF is on

In new router the preprocess() method will be called no mater if SEF On or Off.
If I right remember.

@Fedik, I was unable to get 'preprocess()' to work correctly using your sample in the other issue you mentioned without breaking some other links in the menu so I am just going to have to manage the extra lines of code I have added to buildInit until I can move to the newer router approach while running it under J4. Not ideal, but that is where I am at if I want to use Joomla 4.

I was able to find this reference to the change, #12168, back in 2016 and there it is acknowledged it will cause a B/C issue but that hasn't made it through to the more recent documentation available.

I am okay with this being closed as expected behaviour.

I was unable to get 'preprocess()' to work correctly using your sample in the other issue you mentioned without breaking some other links

I think for your case, to mimic old behavior it need more simple than in my example.
Kind of this:

public function preprocess(&$query)
{
  if (!isset($query['Itemid'])) 
    {
      $query['Itemid'] = Factory::getApplication()->input->getInt('Itemid');
    }
}

It will always add current Itemid if there no one exists.

It not a right way to do routing, but may help for your case.

closed as expected