[SuperEditor] Fix Structured Content Paste Edge Case (Resolves #2626)

[AlabasterAxe]

Previously, when pasting mergable multi-paragraph content at the beginning of an empty document, the first paragraph would get dropped. This was because the code was merging the content of the initial paragraph with the blank initial ParagraphNode. The code would then check if the original node was empty to determine if it should be removed but at that point it had been replaced in the document with the content of the initial pasted paragraph.

This pr just refetches the selection node by the supplied node id and uses that to determine if the initial paragraph needs to be removed.

#2626

Tests

• existing paste tests pass
• added new test for this case

[matthew-carroll]

@AlabasterAxe can you clarify what the user-facing issue is? I couldn't figure it out based on your description.

Are you saying that an extra paragraph exists after paste? Or an expected paragraph disappears? Or an exception is thrown?

[AlabasterAxe]

Yeah, the first paragraph in the pasted content would disappear.

[AlabasterAxe]

I think the golden test failures are unrelated and the unit test failures above happened because the test was not in the same package as the fix so I moved the test to the super_editor package but let me know if there is a more preferable place for it.

[matthew-carroll]

@AlabasterAxe to be crystal clear, when you say...

Yeah, the first paragraph in the pasted content would disappear.

You're saying that if I paste the following:

Hello - paragraph one
World - paragraph two

The resulting document will be:

World - paragraph two

[AlabasterAxe]

I think reading the test will be helpful if you're looking for exact precision of the circumstances under which this problem arises but here's the order of operations:

Initial State: Document with a single ParagraphNode in it with the empty string

Action: Execute PasteStructuredContentEditorCommand with a Document with Two ParagraphNodes ("Hello", "World") at offset 0 into the existing paragraph node.

Expected State:

Document with Two ParagraphNodes ("Hello", "World")

Actual State:

Document with a Single ParagraphNode ("World")

Note that this modification to the document happens programmatically so I'm not saying that this problem would be reproducible via an end user pasting.

[AlabasterAxe]

@matthew-carroll Is there anything else that I need to do for this PR?

Just to explain a bit more about why this happens, it seems like this is an artifact of the node merging logic. If the blockType of the nodes match, first paragraph of the pasted content is inserted into the node where the user's caret is. Otherwise, the first node of the content is inserted into the document after the node with the caret.

This has an unintended side effect where the user had their caret in an empty paragraph initially. If the content is not merged with the node then there is a left-over initial paragraph which would show up as a blank line where the user pasted. To avoid that case, the code checks the initial paragraph where the user pasted to see if it is empty, in which case that node is removed.

The problem arises when the first node of the pasted content is mergable with caret node and the caret node is empty. The content is merged into the initial node by replacing the node in the document. The code then uses the now-stale node object to determine whether the caret node should be removed.

It sees that the original node was empty originally and removes it, deleting the content of the initial paragraph along with it.

Let me know if there's more tests you think I should add or if you have any feedback on the code!