Corrupted error messages

Type: Bug

## Issue Description ##
When I make code changes to a .cshtml file, any minor text change, the linter flags errors on nearly every line. If I restart VS Code the error flags clear. For example, I added a block and now text above the block and below are showing random red error squiggle lines.

## Steps to Reproduce ##
The file contains HTML and Razor shorthdand. I add an HTML element and the error flags fill the page.

## Expected Behavior ##
No error flags on valid code.

## Actual Behavior ##
Error flags on valid code.

Extension version: 2.140.9
VS Code version: Code 1.136.2 (88e44fa0e00b08f7758b4f6d05632e4fd5e4df6f, 2026-09-04T21:40:42Z)
OS version: Windows_NT x64 10.0.26200
Modes:

### Logs ###

### C# log ###

```
2026-09-08 14:09:52.491 [info] Locating .NET runtime version 10.0.5
2026-09-08 14:09:53.844 [info] Dotnet path: C:\Program Files\dotnet\dotnet.exe
2026-09-08 14:09:53.844 [info] Activating C# + C# Dev Kit...
2026-09-08 14:09:54.559 [info] info: Program[0]
2026-09-08 14:09:54.559 [info] Server started with process ID 34868
2026-09-08 14:09:55.289 [info] [Program] Language server initialized
2026-09-08 14:09:55.290 [info] [Program] Monitoring client process 30596 for exit
2026-09-08 14:09:55.707 [info] [initialized] [Microsoft.CodeAnalysis.LanguageServer.FileBasedPrograms.FileBasedProgramsEntryPointDiscovery] Finished discovery in 'c:\Users\e174835\Documents\customapps-repo\OpsLog.NET' in 7 milliseconds
2026-09-08 14:09:59.154 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project C:\Users\e174835\Documents\customapps-repo\OpsLog.NET\OpsLog.csproj loaded by C# Dev Kit
2026-09-08 14:11:01.889 [error] Request textDocument/formatting failed.
Message: 'end' must not be less than 'start'. start='959' end='958'. (Parameter 'end')
Code: -32000
[object Object]

```

### C# LSP Trace Logs log ###

```

```

## Environment Information ##
**VSCode version**: 1.136.2
**C# Extension**: 2.140.9
**Using OmniSharp**: false

Dotnet Information
.NET SDK:
Version: 10.0.302
Commit: 35b593bebf
Workload version: 10.0.300-manifests.714b12c0
MSBuild version: 18.6.11+35b593beb

Runtime Environment:
OS Name: Windows
OS Version: 10.0.26200
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\10.0.302\

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
Version: 10.0.10
Architecture: x64
Commit: f7d90799ce

.NET SDKs installed:
8.0.420 [C:\Program Files\dotnet\sdk]
10.0.302 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.26 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.10 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.26 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.10 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.26 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.10 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
DOTNET_CLI_UI_LANGUAGE [en-US]
DOTNET_MULTILEVEL_LOOKUP [0]

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

Visual Studio Code Extensions

|Extension|Author|Version|Folder Name|
|---|---|---|---|
|csdevkit|ms-dotnettools|3.20.199|ms-dotnettools.csdevkit-3.20.199-win32-x64|
|csharp|ms-dotnettools|2.140.9|ms-dotnettools.csharp-2.140.9-win32-x64|
|powershell|ms-vscode|2025.4.0|ms-vscode.powershell-2025.4.0|
|vscode-dotnet-runtime|ms-dotnettools|3.1.0|ms-dotnettools.vscode-dotnet-runtime-3.1.0|;

C# Settings

|Setting|Value|
|---|---|
|preferCSharpExtension|false|
|compilerDiagnosticScope|openFiles|
|analyzerDiagnosticScope|openFiles|
|enableXamlTools|true|
|useServerGC|true|;

System Info

|Item|Value|
|---|---|
|CPUs|13th Gen Intel(R) Core(TM) i7-13700 (24 x 2112)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x8086, DEVICE=0xa780 [Intel(R) UHD Graphics 770], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.7077 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.8972
Machine model name:
Machine model version:
direct_rendering_display_compositor: disabled_off_ok
gpu_compositing: enabled
multiple_raster_threads: enabled_on
opengl: enabled_on
rasterization: enabled
raw_draw: disabled_off_ok
skia_graphite: disabled_off
trees_in_viz: disabled_off
video_decode: enabled
video_encode: enabled
webgl: enabled
webgpu: enabled
webnn: disabled_off|
|Load (avg)|undefined|
|Memory (System)|15.70GB (2.69GB free)|
|Process Argv|--crash-reporter-id 298a3818-57d5-4a4b-b81c-3ecd8a0533e3|
|Screen Reader|no|
|VM|0%|

A/B Experiments

```
vsliv368:30146709
vswsl492:30256859
binariesv615:30325510
nativeloc1:31344060
dwcopilot:31170013
dwoutputs:31242946
copilot_t_ci:31333650
e5gg6876:31282496
pythonrdcb7:31342333
6518g693:31463988
aj953862:31281341
envsactivate1:31551504
478i5457:31544283
cloudbuttont:31379625
3efgi100_wstrepl:31403338
ec5jj548:31422691
cp_cls_t_966_ss:31526232
4je02754:31466945
c3h7c220:31478652
cp_cls_t_1081:31454832
conptydll_true:31498968
e9c30283:31461165
46204921:31447328
ei9d7968:31496641
chat:31457767
8hig5102:31480529
89g7j272:31518289
i2gc6536:31499202
52612955:31516516
23c7c724:31520045
ddid_t:31478206
hmra_i5g22:31518061
7df3h592:31512476
cp_cls_t_1082:31535311
logging_enabled_new:31498466
db5d2638:31499441
jb_cp_cls_t_632:31543129
56dj4588:31512888
32d76977:31512328
ha629193:31508444
a1ije391_t:31540920
jbcp_cls_pctr_t:31531130
cp_intellij_t_nes:31548657
jf4hg949:31526829
ahp-both-windows:31556933
ihg5j128:31534457
7g2b5551:31542111
nes-ex-cf-euporie-03:31542868
enable_editor_pane_layout:31569726
allow-none:31555437
ac562147:31564510
c7c27ce7:31554789
1h923230:31564177
0dg73464:31554319
treatment-23-1:31555779
unuse_dynamic_mcp:31555281
0d8dfbc3:31570291
0c1h4866:31566224
session-archive:31558132
5b8j3302:31564601
mangle-name-treatment:31572634
intellij_nes_preview_t:31580086
39a5d156:31574986
2bfb8b39:31576696
permission_prompt_control:1332567
ccr_pr_nudge_control:1319470
vsc_wsm_t:1340632
jidc7660:1370295
auto_mode_control:1367906
h5e14506_copy:1348995
1g5a5742:1379227

```

Start by reproducing the issue in a .cshtml file after adding an HTML element, then inspect the C# log's failed textDocument/formatting request with the reported start/end positions. The work is done when valid Razor and HTML edits no longer produce widespread false diagnostics or formatting errors without restarting VS Code.

Written by the indexing model from the issue text.