[folding] custom folding text for folded ranges

[ris58h]

Other IDE's like Visual Studio or Intellij IDEA allow to define custom text for folded regions.

There are a lot of issues that could be solved with this feature: microsoft/vscode#70794 , microsoft/vscode#3352 , #827 and other.

The proposed change is to add collapsedText property to FoldingRange interface:

interface FoldingRange {
	startLine: uinteger
	startCharacter?: uinteger
	endLine: uinteger;
	endCharacter?: uinteger;
	kind?: string;

        collapsedText?: string;
}

The problem is: what should clients that don't support char-ranges folding (lineFoldingOnly === true) do? I've described the problem in microsoft/vscode#70794
Possible options are:

1. Clients must replace the whole first line too.
2. Clients must show the collapsedText at the end of the first line.
3. Clients are free to decide how they show it.
4. Add a property to the FoldingRange to specify the behavior.

[aeschli]

Having a collapsedText makes sense.
Following the current logic, for lineFoldingOnly === true, by default it will be shown at the end of the line.

To replace the full line, we could allow that the startCharacter can be set to 0, even when lineFoldingOnly === true.

[ris58h]

@aeschli it's a great idea to use startCharacter 👏
Do we need an additional property in FoldingRangeClientCapabilities to specify that client doesn't support collapsedText?

[aeschli]

Yes, a new capability is needed. It will be false by default.

[ris58h]

Ok. What's next? Should I make PR in https://github.com/microsoft/vscode-languageserver-node repository?

[dbaeumer]

Yes, first we need to have an implementation for this.

[ris58h]

The PR is ready microsoft/vscode-languageserver-node#910

[dbaeumer]

@ris58h we would need a PR against the specification as well. Can you provide one so that this gets properly documented?

[ryzngard]

@ris58h are you able to do a PR against the specification here? Definitely interested in this moving forward.

[ris58h]

@ryzngard I'm interested too but I'm on vacation right now, so not in the very near future.

[ryzngard]

I went ahead and created a PR in the meantime, which I hope is correct. Please let me know if anything needs to change.

@ris58h if you get back from vacation and want to take over by all means feel free.

[dbaeumer]

Added to 3.17.