Paragraph shouldn't be pasted into the list item

[Mgsy]

🐞 Is this a bug report or feature request? (choose one)

• Bug report

💻 Version of CKEditor

1.0.0-alpha2

📋 Steps to reproduce

1. Go to http://localhost:8125/ckeditor5-core/tests/manual/article.html.
2. Above Heading 1 create new block quote with paragraph inside.
3. Select paragraph from the block quote and part of the Heading 1.
4. Copy it.
5. Select UL List item 1 and UL List item 2.
6. Paste.

✅ Expected result

Content should be pasted properly.

❎ Actual result

<p> element has been pasted to the <li> element.

📃 Other details that might be useful

GIF

Other information
OS: Windows 10, MacOS X
Browser: All browsers

This bug was reproducible before the engine refactoring.

[Reinmar]

This is bad. Not a regression, but still important because it will lead to crashes.

[Reinmar]

This is a tricky case.

We start with:

<listItem>fo[]o</listItem>

And insert:

<blockQuote>
    <paragraph>xxx</paragraph>
</blockQuote>
<heading1>yyy</heading1>

At some point of the insertContent() method we have this:

<listItem>fo</listItem>
<blockQuote>
    <paragraph>xxx</paragraph>
</blockQuote>
<listItem>o</listItem>

So, it's a valid content. But then, we came to a point when we want to merge the block in which insertion started with the first element of the content that was inserted. Normally it works fine – if there was no <blockQuote> but only the <paragraph>xxx</paragraph> element we'd get:

<listItem>foxxx</listItem>
<heading1>yyy[]o</heading1>

However, due to the <blockQuote> we get this:

<listItem>fo<paragraph>xxx</paragraph><listItem>
<heading1>yyy[]o</listItem>

---

I've known about these cases for a long time but ignored them due to their complexity and because we didn't have bQs back then (so that case was not possible to reproduce).

Right now, we need to do something that will prevent incorrect merges. The easiest solution I can think if is to not merge when that might lead to an incorrect content, so a new method Schema#checkMerge() (see https://github.com/ckeditor/ckeditor5-engine/issues/1240).

The result will be then:

<listItem>fo</listItem>
<blockQuote>
    <paragraph>xxx</paragraph>
</blockQuote>
<heading1>yyy[]o</heading1>

Which I think is reasonable. There might be better solutions in terms of final result, but none seem to be feasible.