Steps to reproduce the issue

I am building a component managing a set of job offers. Jobs have attributes including "employer".

There is a front-end list view with filtering that allows users to restrict the listed jobs to a particular employer (among other filtering options). Selecting an employer filter causes the following GET request to be generated:

http://localhost/joomla/index.php?option=com_jobs&view=jobs&filter[employer]=Google

This works nicely. However, now I wish to create a menu item type with an option, the employer, so I can create menu items, such as, Google Jobs, Apple Jobs, etc.. Essentially, I wish the menu items to preset the filter with a particular value that can be specified during menu item definition.

Ok. So I added a filter field group to the menu item type definition (joomla/components/com_jobs/views/jobs/templ/default.xml):

    <?xml version="1.0" encoding="utf-8"?>
<metadata>
	<layout title="COM_JOBS_TITLE_LIST_VIEW_JOBS_TEST" option="View">
		<message>
						<![CDATA[COM_JOBS_TITLE_LIST_VIEW_JOBS_DESC]]>
		</message>
	</layout>
	<!-- Add fields to the request variables for the layout. -->
	<fields name="request">
		<fieldset name="request">
			<fields name="filter">
				<field name="employer"
				  type="text" 
				  hint="COM_JOBS_FILTER_EMPLOYER_HINT"
				  label="COM_JOBS_JOBS_EMPLOYER"
				  class="js-stools-search-string" />
			</fields>
		</fieldset>
	</fields>
</metadata>

Expected result

When I create a menu item of this type and I enter Google into the employer field and save the menu item, the following link should be created for it:

index.php?option=com_jobs&view=jobs&filter[employer]=Google

Actual result

index.php?option=com_jobs&view=jobs

Moreover, after saving, the employer field becomes empty.

System information (as much as possible)

PHP erstellt für 	Linux Compluter5 4.15.0-51-generic #55-Ubuntu SMP Wed May 15 14:27:21 UTC 2019 x86_64
Datenbanktyp 	mysql
Datenbankversion 	5.5.5-10.3.13-MariaDB-1:10.3.13+maria~xenial
Datenbankzeichensatz 	utf8_general_ci
Datenbankverbindungszeichensatz 	utf8mb4_general_ci
PHP-Version 	7.2.19-0ubuntu0.18.04.1
Webserver 	Apache/2.4.29 (Ubuntu)
PHP-Interface für den Webserver 	apache2handler
Joomla!-Version 	Joomla! 3.9.5 Stable [ Amani ] 9-April-2019 15:00 GMT
Joomla!-Plattform-Version 	Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
Browsererkennung 	Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0 

Additional comments

The problem occurs only if the filter field group is the first element (or only element?) in the request fieldset. A workaround is to insert a hidden field like so:

	<fields name="request">
		<fieldset name="request">

			<!-- Needed because of Joomla bug: fields group cannot be first in the fieldset -->
			<field name="xxx" type="hidden" />

			<fields name="filter">
				<field name="employer"
				  type="text" 
				  hint="COM_JOBS_FILTER_EMPLOYER_HINT"
				  label="COM_JOBS_JOBS_EMPLOYER"
				  class="js-stools-search-string" />
			</fields>
		</fieldset>
	</fields>

A possible fix is to set the $nested parameter of getGroup to true in administrator/components/com_menus/controllers/item.php at about line 360:

	// Preprocess request fields to ensure that we remove not set or empty request params
-	$request = $form->getGroup('request');
+	$request = $form->getGroup('request', true);

With this change, $request will no longer be empty. This solves the problem but I am not sure what other unintended effects the change might have.