When nested folding ranges end on the same line, the outer range is incorrectly truncated

Describe the bug
Switch case folding doesn't fold until the next case if a function has parameters that span multiple lines (I think that is the issue, but maybe something similar at least?).

To Reproduce
Add a file with this sample:

Future<void> f() async {
  switch (1) {
    case 1:
      print('something');

      someBigFunctionName(
        someReallyReallyLongNameHereSoItHasToSplit: 1234567,
      );
    case 2:
      print('another thing');
      print('yet another thing');
    case 3:
      print('more things');
  }
}

void someBigFunctionName({
  int? someReallyReallyLongNameHereSoItHasToSplit,
}) {}

Try to fold at case 1 and see that it doesn't fold someBigFunctionName.

Expected behavior
It folds someBigFunctionName.

Please complete the following information:

Workspace Environment

Dart Code extension: 3.121.20250930
Flutter extension: 3.121.20250930 (activated)

App: Visual Studio Code
App Host: desktop
Version: win 1.105.0

Workspace type: Dart (LSP)
Workspace name: bug

Dart (3.11.0-edge.1974c2360e1ee4345b9cb15929b1a2465559dc18): C:\Users\felip_0vh5fa6\dart
Flutter (3.35.5): C:\Users\felip_0vh5fa6\.puro\envs\stable\flutter (No device)

Output from 'dart info'

C:\Users\felip_0vh5fa6\dart\bin\dart.exe info

If providing this information as part of reporting a bug, please review the information
below to ensure it only contains things you're comfortable posting publicly.

General info

Dart 3.11.0-edge.1974c2360e1ee4345b9cb15929b1a2465559dc18 (main) (Mon Oct 6 18:46:13 2025 -0700) on "windows_x64"
on windows / "Windows 11 Pro" 10.0 (Build 26100)
locale is pt-BR

Project info

sdk constraint: '^3.9.0'
dependencies: analyzer, bloc, meta
dev_dependencies: lints, test, test_reflective_loader, very_good_analysis

Process info

Memory	CPU	Elapsed time	Command line
0 MB	--		dart.exe
0 MB	--		dart.exe
0 MB	--		dart.exe
0 MB	--		dart.exe
0 MB	--		dart.exe
0 MB	--		dart.exe
0 MB	--		dart.exe
0 MB	--		dart.exe
0 MB	--		dart.exe
0 MB	--		dartvm.exe
0 MB	--		dartvm.exe
0 MB	--		dartvm.exe
0 MB	--		dartvm.exe

Operating System and version:
VS Code version:
Dart extension version:
Dart/Flutter SDK version:
Target device (if the issue relates to Flutter debugging):

I've tried adding some tests to the same files you edited for https://dart-review.googlesource.com/c/sdk/+/431281, but I could not create a test that would fail even with the above example:

pkg/analysis_server/test/src/computer/folding_computer_test.dart

  Future<void> test_switchCase() async {
    var content = '''
Future<void> f/*[0*/(int i) async {
  switch (i) {/*[1*/
    case 1:/*[2*/
      print('something');

      someBigFunctionName(/*[3*/
        someReallyReallyLongNameHereSoItHasToSplit: 1234567,
      /*3]*/);/*2]*/
    default:/*[4*/
      break;/*4]*/
  }/*1]*/
}/*0]*/

void someBigFunctionName/*[5*/({
  /*[6*/int? someReallyReallyLongNameHereSoItHasToSplit,
}/*6]*/) {}/*5]*/
''';
    await _computeRegions(content);
    expectRegions({
      0: FoldingKind.FUNCTION_BODY,
      1: FoldingKind.BLOCK,
      2: FoldingKind.BLOCK,
      3: FoldingKind.INVOCATION,
      4: FoldingKind.BLOCK,
      5: FoldingKind.FUNCTION_BODY,
      6: FoldingKind.PARAMETERS,
    });
  }

pkg/analysis_server/test/lsp/folding_test.dart

  Future<void> test_switchCase() async {
    var content = '''
Future<void> f/*[0*/(int i) async {
  switch (i) {/*[1*/
    case 1:/*[2*/
      print('something');

      someBigFunctionName(/*[3*/
        someReallyReallyLongNameHereSoItHasToSplit: 1234567,
      /*3]*/);/*2]*/
    default:/*[4*/
      break;/*4]*/
  }/*1]*/
}/*0]*/

void someBigFunctionName/*[5*/({
  /*[6*/int? someReallyReallyLongNameHereSoItHasToSplit,
}/*6]*/) {}/*5]*/
''';
    await _computeRegions(content);
    expectRegions({
      0: FoldingKind.FUNCTION_BODY,
      1: FoldingKind.BLOCK,
      2: FoldingKind.BLOCK,
      3: FoldingKind.INVOCATION,
      4: FoldingKind.BLOCK,
      5: FoldingKind.FUNCTION_BODY,
      6: FoldingKind.PARAMETERS,
    });
  }

@FMorschel

@FMorschel I think the second test you pasted is incorrect (just the same as the first?) and doesn't compile.

My first guess would be that this is related to line folding.. After computing the folding ranges, we have to do some work to merge/exclude some if the client only supports line-folding (which VS Code does) - see _compensateForLineFolding. If it is, I think it's a bug though, because on the surface I don't see why we wouldn't produce the outer range as you'd expect.

(I assume you're looking at this, but if you want me to, just let me know!)

I ran both tests locally, and they passed. I'm not sure what you meant.

One thing I noticed was, if you close that section and reload the VS Code window (Developer: Reload Window), it shows as fully folded until the extension and server start. At that point, it automatically moves out the invocation outside the folded region.

I tried looking at this, yes, but since I had no luck reproducing on a test, you may have a better chance of figuring this one out, so please continue this, but feel free to ask for more guidance if you need any. Or if you're having trouble reproducing this.

I ran both tests locally, and they passed. I'm not sure what you meant.

I think you copied the same test twice into your comment above - the one you marked as LSP doesn't compile in the LSP file.

But if you're not looking at it, I'm happy to look, thanks!

Oh, yeah, that's probably it hahaha. Here is the correct test:

  Future<void> test_switchPattern_2() async {
    var content = '''
Future<void> f(int i) async {
  switch (i) {/*[0*/
    case 1:/*[1*/
      print('something');

      someBigFunctionName(/*[2*/
        someReallyReallyLongNameHereSoItHasToSplit: 1234567,
      /*2]*/);/*1]*/
    default:/*[3*/
      break;/*3]*/
  }/*0]*/
}

void someBigFunctionName({
  int? someReallyReallyLongNameHereSoItHasToSplit,
}) {}
''';

    await computeRanges(content);
    expectRanges({
      0: noFoldingKind,
      1: noFoldingKind,
      2: noFoldingKind,
      3: noFoldingKind,
    }, requireAll: false);
  }

Off-by-one error 😐

Got a fix at https://dart-review.googlesource.com/c/sdk/+/454920

folding.mp4

[analysis_server] Fix folding ranges when nested ranges end on the sa…

Fixed by dart-lang/sdk@ceb6b5f, ships in the SDK :-)

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When nested folding ranges end on the same line, the outer range is incorrectly truncated #5750

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When nested folding ranges end on the same line, the outer range is incorrectly truncated #5750

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FMorschel commented on Oct 13, 2025

DanTup commented on Oct 14, 2025

DanTup commented on Oct 14, 2025

FMorschel commented on Oct 14, 2025

DanTup commented on Oct 14, 2025

FMorschel commented on Oct 14, 2025

DanTup commented on Oct 14, 2025

DanTup commented on Oct 16, 2025

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