[BUG] Kirby Page memory leak

[RasmusKjeldgaard]

• I have written a good issue

Describe the bug

During a performance audit of DRB a memory leak in Kirby Page was discovered.
When navigating between pages, the Kirby Page is not properly detached when the component is no longer present. This results in an ever increasing amount of DOM nodes kept in the browser, that is not cleaned up by garbage collection and might eventually result in a hard crash, and otherwise just poor performance.

The problem is easily illustrated with two simple pages:
https://user-images.githubusercontent.com/42470636/193853109-5eb944b3-1e20-4eca-bec9-edc26bbada00.mov

As the video shows, the second pages DOM nodes becomes detached and are still in memory when the page is destroyed no longer shown.

Describe how to reproduce the bug

1. Go to develop
2. Spin up the flows app with npm run start:flows
3. Click on the 'Navigate to list item detail' flow
4. See how pages are kept in memory in via the Performance monitor in Chrome

Which Kirby version was used?

7.0.0

What was the expected behavior?

We want to see the DOM nodes removed when a page component is popped/removed from the app.

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Checklist:

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Verification

To make sure the bug is not intended behaviour; it should be verified by a member of team Kirby before moving on to implementation.

• Make sure the NOT verified label has been removed by a member of team Kirby; do not proceed until this is done

Implementation

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• Make sure you have read: "Before you get coding".
• Signal to others you are working on the issue by assigning yourself.
• Create a branch from the develop branch following our branch naming convention.
• Create a test that reproduces the bug following guidelines in: "The good: Test".
• Write code to fix the bug

Review

Once the issue has been implemented and is ready for review:

• Do a self-review.
• Create a pull-request.