Allow to disable new line padding on paste for specific elements

[Mati365]

Suggested merge commit message (convention)

Fix (clipboard): Pasting an inline widget as plain text no longer inserts additional empty lines before and after pasted widget. Closes #17052

MAJOR BREAKING CHANGE (clipboard): Pasting an inline widget as plain text no longer inserts additional empty lines before and after pasted widget.

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Additional information

Not sure if proposed solution is the correct one. How can we determine model element schema attributes based on view elements from clipboard content?

Before:

before-new-line-fix-2024-09-10_08.51.47.mp4

After:

after-new-line-fix-2024-09-10_08.50.11.mp4

[Witoso]

Why this works on our merge fields and doesn't on custom written placeholders? Maybe we lack some abstraction?

[Mati365]

@Witoso From what I can see merge tags use dataPipeline:transparentRendering to handle this, but in our case it's not working properly. It's not upcasting placeholders and keeps raw text after setting this attribute on placeholder element specified in example.

@Dumluregn Do you know why this happens? Can we use dataPipeline:transparentRendering without breaking upcasting?

[niegowski]

The newLinePadding() helper is unfortunately based on the assumption that the view ContainerElement is a block element and this is not a valid assumption. Maybe we could use DomConverter#blockElements as a list of view elements which should be considered a block?

I'd suggest not adding another custom property.

[Mati365]

@niegowski Isn't it breaking change? I'm not sure what should API look like, because we have plenty of downcasts which do not determine which view element is a block element and which not.

ahh, now I get it. I was thinking that we want to add new blockElements property. nvm

[Mati365]

@niegowski Looks like it works 🎉

[niegowski]

The code looks ok but the breaking change description must be updated to reflect the code change. The signature of that helper changed and since it's a public helper it could be used in some external integrations.

[Witoso]

Please, let's plan this BC together for some specific release. Don't merge.