Fix: Flickering page on route navigation

[callingmedic911]

Closes #2124.

Why was this happening?
PageLoader was getting unmounted on each route change. And when it remounts it switches to the default state - page is undefined. The cause of unmounting is from activeRouteTree (screenshot 1) in router.tsx. It swaps out the Route, so the whole subtree is unmounted.

This PR is the possible fix that updates the LocationAwareRouter to maintain a single route element and copy all attributes/props from the active route. So, react can, well, react.

Things to consider now:

• Is this a good approach to fix this? EDIT: Tests are failing, clearly I missed something. Will look into it. EDIT2: Using different approach now.
• Do we need to copy over anything else apart from props, children or other properties of activeRoute element?
• Finalize approach for testing.

[jtoar]

Awesome find @callingmedic911! The beta React docs talk a lot about the role tree structure plays in React's rendering/committing: https://beta.reactjs.org/learn/preserving-and-resetting-state. The new docs are really great.

A simple test for this may be checking that the tree structure stays exactly the same between renders (except for the Page at the very end).

[callingmedic911]

Seems like the fix won't be straightforward like this because activeChildren is not always a Route. It could be n-level nested with Set.

Checking how we can improve here.

[callingmedic911]

Trying this new approach, where unmount still happens but now the Router context holds current page module, so it instead of undefined it resets old module. So, at least white flash doesn't appear.

Long term fix can be a bit of refactoring. Theoretically, we have to put Loader outside of Route/activeChildren and pass download page module as a prop to the current Route. Then instead of returning PageLoader from InternalRoute, it can return Page directly. But this seemed too much code change considering we are very near to v1. However, if we agree to it I can proceed with guidance from you guys - @Tobbe @jtoar.

[Tobbe]

@callingmedic911 Please have a look at this commit and see what you think of my alternative solution for this issue Commit #2 for PR #3772

[callingmedic911]

Closing in favour of #3772.