Make custom pasting less eager

[fbricon]

In JBang VS Code, I have my own paste provider to convert Maven XML dependencies into JBang //DEPS. Now that vscode-java provides its own provider, it takes over and makes the experience less pleasant:

IMO, vscode-java should be more clever and do not return a DocumentPasteEdit if no special handling is necessary (eg. pasted content is not java, or pasting site is not within String quotes.

[rgrunber]

Interestingly, the vscode-java contributed paste edit isn't always present. If I paste xml within the type declaration, the vscode-java paste edit is never created. However, when done in the import area, it is. I think this might actually be a bug in the language server.

null is returned when pasting within the type declaration :

https://github.com/eclipse-jdtls/eclipse.jdt.ls/blob/2b7857b1fbc3ff35f7c126c6e7dadb53665977e9/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/PasteEventHandler.java#L336-L338

the original paste text is returned in a lot of "failing" cases, or if we're not within a type declaration :

https://github.com/eclipse-jdtls/eclipse.jdt.ls/blob/2b7857b1fbc3ff35f7c126c6e7dadb53665977e9/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/PasteEventHandler.java#L286-L301

So we probably need to try returning null everywhere, and that probably fixes it.

vscode-java/src/pasteEventHandler.ts

Lines 103 to 104 in b8fd158

	// either the handler returns null or encounters problems, fall back to return undefined to let VS Code ignore this handler
	return undefined;

The client is already set up to ignore the handler if the delegate doesn't return anything.

[rgrunber]

@hopehadfield , interested in this ? Should be easy to fix, but just requires a bit of verifying the desired behaviour.

[rgrunber]

@hopehadfield , are you able to reproduce the behaviours described in (without the PR that fixed this) :

#3445, #3236, #3237, #3164 . Does this fix them ?

Update: I think it does! 😎

[hopehadfield]

@rgrunber I could reproduce everything except for the issues relating to multi-cursor paste (those seemed to work fine for me even without this PR). This fixed all the other issues for me though!