Editor: Flatten Inserter mapSelectToProps to optimize rendering

[aduth]

This pull request seeks to optimize the rendering of the Inserter component by avoiding the return of a new reference object which had been returned previously in each call to its withSelect mapSelectToProps. It does so by merely flattening the insertionPoint object it had been creating.

As a general rule of thumb, mapSelectToProps should never create new objects used in values of its return object.

This has the impact of reducing the number of render calls on Inserter for a new post from ~50 to 4.

Testing instructions:

This is a refactoring change. There is expected to be no effective changes.

Ensure unit tests pass.

npm run test-unit packages/editor/src/components/inserter/test/index.js

Verify basic usage of the inserter.

[youknowriad]

The two lines above are different from what we had previously. As we were not assigning the insertionPoint.rootClientId before testing. Not sure if it's impactful.

[aduth]

Oh, you're totally right. I expect this is wrong as implemented. I'll plan to fix.

[youknowriad]

I think this should be initiailized something like getBlockOrder( rootClientId ).length?

[aduth]

As far as I can tell, it's not needed, for reasons described in the comment a few lines below (maybe more valuable to pull up?)

// Otherwise, the default behavior for an undefined index is to
// append block to the end of the rootClientId context.

[aduth]

Related:

gutenberg/packages/editor/src/store/reducer.js

Lines 422 to 428 in a548961

	const { index = subState.length } = action;
	return {
	...state,
	...mappedBlocks,
	[ rootClientId ]: insertAt( subState, mappedBlocks[ rootClientId ], index ),
	};

[youknowriad]

Feel free to merge when tests pass 👍