Compile_commands.json not work if preprocessor changed before

Environment

• OS and Version: window 23H2 wsl2 unbuntu 22.04 lts
• VS Code Version: version 1.106
• C/C++ Extension Version: 1.28.3
• If using SSH remote, specify OS of remote machine:

Bug Summary and Steps to Reproduce

Bug Summary: IntelliSense inactive regions reset (flicker) to active state on file save when using compile_commands.json in WSL.

Steps to reproduce:

Setup a C project in WSL2 using the Microsoft C/C++ extension and generate a compile_commands.json (e.g., using bear).

Configure .vscode/c_cpp_properties.json to use "compileCommands": "${workspaceFolder}/compile_commands.json" and strictly set "includePath": [] (empty).

The project supports multiple build targets. The preprocessor definitions in compile_commands.json dynamically switch between -DTARGET_IMX93 and -DTARGET_IMX8MP for the same source file depending on the selected configuration.

Build for one target (e.g., TARGET_IMX8MP) so that compile_commands.json is updated with -DTARGET_IMX8MP.

Open a source file (.c) containing conditional preprocessor blocks (#ifdef / #else) where one block should be inactive (dimmed) based on the defines in compile_commands.json.

Observe that the inactive region is correctly dimmed initially.

Press Ctrl+S to save the file.

See error: The dimmed inactive region immediately becomes active (opaque/bright) again, ignoring the compile_commands.json configuration.

Additional Context (Comparison with Clangd extension): The vscode-c/c++ (cpptools) extension fails to maintain the state, but the clangd extension works perfectly. When I disable the Microsoft C/C++ extension and use the clangd extension with the exact same compile_commands.json and project structure, the inactive regions are handled correctly and persist even after saving.

c_cpp_properties.json

{
    "configurations": [
        {
            "name": "Linux",
            "includePath": [],
            "compileCommands": "${workspaceFolder}/compile_commands.json",
            
            "cStandard": "c17",
            "cppStandard": "c++17"
        }
    ],
    "version": 4
}

Configuration and Logs

Diagnostics

-------- Diagnostics - 12/3/2025, 5:42:20 PM
Version: 1.28.3
Current Configuration:
{
    "name": "Linux",
    "includePath": [],
    "compileCommands": [
        "/home/leejunggyun/workplace/test/multi_source/compile_commands.json"
    ],
    "cStandard": "c17",
    "cppStandard": "c++17",
    "compilerPathIsExplicit": false,
    "cStandardIsExplicit": true,
    "cppStandardIsExplicit": true,
    "intelliSenseModeIsExplicit": false,
    "recursiveIncludesReduceIsExplicit": false,
    "recursiveIncludesPriorityIsExplicit": false,
    "recursiveIncludesOrderIsExplicit": false,
    "compileCommandsInCppPropertiesJson": [
        "${workspaceFolder}/compile_commands.json"
    ],
    "mergeConfigurations": false,
    "recursiveIncludes": {},
    "browse": {
        "limitSymbolsToIncludedHeaders": true
    }
}
Additional Tracked Settings:
{
    "editorTabSize": 4,
    "editorInsertSpaces": true,
    "editorAutoClosingBrackets": "languageDefined",
    "filesEncoding": "utf8",
    "filesAssociations": {
        "menu_device_imx8mp.h": "c",
        "menu_device_imx93.h": "c"
    },
    "filesExclude": {
        "**/.git": true,
        "**/.svn": true,
        "**/.hg": true,
        "**/.DS_Store": true,
        "**/Thumbs.db": true
    },
    "filesAutoSaveAfterDelay": false,
    "editorInlayHintsEnabled": true,
    "editorParameterHintsEnabled": true,
    "searchExclude": {
        "**/node_modules": true,
        "**/bower_components": true,
        "**/*.code-search": true
    },
    "workbenchSettingsEditor": "ui"
}
cpptools version (native): 1.28.3.0
Current database path: /home/leejunggyun/.cache/vscode-cpptools/07d841cfe44854793ff5abba64262a39/.browse.VC.db
Translation Unit Mappings:
[ /home/leejunggyun/workplace/test/multi_source/main.c - source TU]:
Translation Unit Configurations:
[ /home/leejunggyun/workplace/test/multi_source/main.c ]
    Process ID: 27178
    Memory Usage: 21 MB
    Compiler Path: /usr/bin/gcc
    Include paths:
        include: /home/leejunggyun/workplace/test/multi_source/menu/include
        system include: /usr/lib/gcc/x86_64-linux-gnu/11/include
        system include: /usr/local/include
        system include: /usr/include/x86_64-linux-gnu
        system include: /usr/include
    Defines:
        TARGET_IMX93
    Standard Version: c17
    IntelliSense Mode: linux-gcc-x64
    Other Flags:
        --gcc
        --gnu_version=110400
    compile_commands.json entry:
        directory: /home/leejunggyun/workplace/test/multi_source
        file: /home/leejunggyun/workplace/test/multi_source/main.c
        arguments:
            /usr/bin/gcc
            -Imenu/include
            -Wall
            -DTARGET_IMX93
            -c
            -o
            main.o
            main.c
        output: /home/leejunggyun/workplace/test/multi_source/main.o
Total Memory Usage: 21 MB
Browse Paths from compile_commands.json, from workspace folder: /home/leejunggyun/workplace/test/multi_source
    /home/leejunggyun/workplace/test/multi_source
    /home/leejunggyun/workplace/test/multi_source/menu
    /home/leejunggyun/workplace/test/multi_source/menu/include
    /usr/include
    /usr/include/c++/12
    /usr/include/c++/12/backward
    /usr/include/x86_64-linux-gnu
    /usr/include/x86_64-linux-gnu/c++/12
    /usr/lib/gcc/x86_64-linux-gnu/11/include
    /usr/lib/llvm-14/lib/clang/14.0.0/include
    /usr/local/include

------- Workspace parsing diagnostics -------
Number of files discovered (not excluded): 8166

compile_commands.json

[
  {
    "arguments": [
      "/usr/bin/gcc",
      "-Imenu/include",
      "-Wall",
      "-DTARGET_IMX93",
      "-c",
      "-o",
      "main.o",
      "main.c"
    ],
    "directory": "/home/leejunggyun/workplace/test/multi_source",
    "file": "/home/leejunggyun/workplace/test/multi_source/main.c",
    "output": "/home/leejunggyun/workplace/test/multi_source/main.o"
  },
  {
    "arguments": [
      "/usr/bin/gcc",
      "-Imenu/include",
      "-Wall",
      "-DTARGET_IMX93",
      "-c",
      "-o",
      "menu/menu_device_imx93.o",
      "menu/menu_device_imx93.c"
    ],
    "directory": "/home/leejunggyun/workplace/test/multi_source",
    "file": "/home/leejunggyun/workplace/test/multi_source/menu/menu_device_imx93.c",
    "output": "/home/leejunggyun/workplace/test/multi_source/menu/menu_device_imx93.o"
  }
]

Other Extensions

No response

Additional context

No response

Reproduce the issue using the WSL2 C project, compile_commands.json, and .vscode/c_cpp_properties.json configuration described in the report. Start by tracing how the compile_commands.json defines are applied before and after saving a source file; done means inactive preprocessor regions remain dimmed after Ctrl+S.