Pull Request for Issue #33307.
All the 6 mentioned problems are fixed by this PR.

Summary of Changes

The $eid variable was assigned it's value by $model->getJoomlaFilesExtensionId()) which always returns the ID of Joomla CMS (212). This was causing the form to behave strangely by not considering the value of the selected extension from the dropdown

Testing Instructions

Step 1: Run this SQL to add a fake post installation message (change the db prefix!)

INSERT INTO `o07n8_postinstall_messages` (`postinstall_message_id`, `extension_id`, `title_key`, `description_key`, `action_key`, `language_extension`, `language_client_id`, `type`, `action_file`, `action`, `condition_file`, `condition_method`, `version_introduced`, `enabled`)
VALUES
	(99, 210, 'TEST', 'TEST', '', 'com_cpanel', 1, 'message', '', '', '', '', '4.0.0', 1);

Step 2: Visit the Post Installation Message screen and follow through:

Before the PR

• Load page, note its Joomla CMS in the dropdown and you see its messages shown
• Select Atum Administrator template from the drop down - see there is one message
• While we are on the "Atum Administrator template" selected page, click Hide all messages button
• Note the page reloads back to the Joomla CMS page and all the Joomla CMS Messages are HIDDEN (Problem 1).
• Select Atum Administrator template from the drop down - see there is one message (Problem 2, it should have been hidden)
• While we are on the "Atum Administrator template" selected page, click Hide this message button
• Note that we are now showing messages for Joomla CMS and not "Atum Administrator template" (Problem 3)
• Select Atum Administrator template from the drop down - see there is no message (Correct).
• Click RESET MESSAGES
• Note that we are now showing messages for Joomla CMS and not "Atum Administrator template" (Problem 4)
• Note that we are now showing messages for Joomla CMS, when we reset messages for "Atum Administrator template" (Problem 5)
• Select Atum Administrator template from the drop down - see there is no message (Problem 6 - they were reset so should be showing!).

After the PR:

Problem 1: Atum's page will be shown on reload and messages will be hidden
Problem 2: Atum's messages won't be visible
Problem 3: Messages shown will be of Atum
Problem 4: Page will reload to display Atum's messages
Problem 5: Reset will only affect Atum's messages
Problem 6: Atum's messages will be displayed

The Problems and fixes mentioned above are shown in the video below.

Actual result BEFORE applying this Pull Request

Expected result AFTER applying this Pull Request

Documentation Changes Required

None