T/1031: Ignore mutations which lead to no changes

[Reinmar]

Suggested merge commit message (convention)

Fix: Mutation observer will ignore children mutations if as a result of several native mutations the element's children haven't changed. Closes ckeditor/ckeditor5#4121.

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Additional information

In

ckeditor5-engine/src/view/renderer.js

Lines 508 to 525 in 4a4a89a

	function sameNodes( actualDomChild, expectedDomChild ) {
	// Elements.
	if ( actualDomChild === expectedDomChild ) {
	return true;
	}
	// Texts.
	else if ( domConverter.isText( actualDomChild ) && domConverter.isText( expectedDomChild ) ) {
	return actualDomChild.data === expectedDomChild.data;
	}
	// Block fillers.
	else if ( isBlockFiller( actualDomChild, domConverter.blockFiller ) &&
	isBlockFiller( expectedDomChild, domConverter.blockFiller ) ) {
	return true;
	}
	// Not matching types.
	return false;
	}

there's a very similar code to the one I wrote. However, my doesn't handle block fillers. There are no block fillers in the view so that seems to be right, but I'm mentioning just in case.

Also, I've been thinking that perhaps both functions should end up in the DomConverter's public interface one day. But not just yet.