This is defined by the "showmycode" setting in the app.json file for your app:
showMyCode No By default, it's set to false and isn't visible in the manifest. To enable viewing the source code when debugging an extension, add the following setting: "showMyCode": true.
Note: This setting will be deprecated in a future release, and replaced by the resourceExposurePolicy setting introduced with runtime 8.0. It's described in this table.
resourceExposurePolicy No Defines the accessibility of the resources and source code during different operations. The resourceExposurePolicy property contains four options for defining access: applyToDevExtension, allowDebugging, allowDownloadingSource, and includeSourceInSymbolFile. All defaults are false. The syntax is "resourceExposurePolicy": {"applyToDevExtension": true, "allowDebugging": true, "allowDownloadingSource": true, "includeSourceInSymbolFile": true}. For more information, see Resource Exposure Policy Setting.
learn.microsoft.com/.../devenv-json-files
