[GHS] custom elements

[niegowski]

Suggested merge commit message (convention)

Feature (html-support): Custom elements should be preserved by GHS. Closes #11432.

Fix (engine): Whitespaces between block elements should not trigger auto-paragraphing. Closes #11248.

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Additional information

[Reinmar]

This breaks the following test:

ckeditor5/packages/ckeditor5-engine/tests/view/domconverter/whitespace-handling-integration.js

Line 110 in 30286f7

it( 'nbsp between blocks is not ignored (between paragraphs)', () => {

We need to choose to either trim that nbsp in that case or correctly handle scenarios like:

<custom>x</custom>

<custom someattr></custom>

where the space (normal space) should be trimmed.

Unfortunately, since these are custom elements we can't easily make the decision on DOM->view conversion whether these are spaces between block or inline elements.

Additionally, unfortunately we lose the information whether this was an nbsp or normal space upon DOM->view conversion. Currently, all are normalized to normal spaces at this stage. if they weren't, the below code would actually correctly differentiate these situation.

Hence, we need to:

• either lose intentional nbsps between blocks (may happen in HTML generated by external tools)
• or make sure custom elements don't break things

[oleq]

Yes.