Get vscode registerCompletionItemProvider to work in a json file with a `word.` trigger

Question

I am using this code to try to register a CompletionProvider in my extension. It is essentially the code from the sample completionProvider sample https://github.com/microsoft/vscode-extension-samples/blob/master/completions-sample/src/extension.ts.

I want it triggered by a . as in "launches." in my extension command in keybindings.json ultimately but it is doing nothing in any json file. Nothing happens, no error.

function activate(context) {

  loadLaunchSettings(context);
  activeContext = context;

  const configCompletionProvider = vscode.languages.registerCompletionItemProvider (
    { language: 'json', scheme: 'file' },   // tried scheme: 'untitled' too
    {
      // eslint-disable-next-line no-unused-vars
      provideCompletionItems(document, position, token, context) {

        // get all text until the `position` and check if it reads `"launches."`

        const linePrefix = document.lineAt(position).text.substr(0, position.character);
        if (!linePrefix.endsWith('\"launches.\"')) {  // tried without the escapes too
          return undefined;
        }

        return [
          new vscode.CompletionItem('log', vscode.CompletionItemKind.Text),
          new vscode.CompletionItem('warn', vscode.CompletionItemKind.Text),
          new vscode.CompletionItem('error', vscode.CompletionItemKind.Text),
        ];
      }
    },
    '.' // trigger
  );

  context.subscriptions.push(configCompletionProvider);
}

In this code:

  {
    "key": "alt+f",
    "command": "launches."   <= provider completion options here
  },

I couldn't find anything helpful and thought I followed the sample closely but no completion suggestions either on typing "launches." or using Ctrl+Space to trigger intellisense.

I do have this setting:

  "editor.quickSuggestions": {
    "comments": true,
    "other": true,
    "strings": true   // <===
  },

And I tried various alternatives presented here to a similar problem: Custom Extension for JSON Completion Does Not Work in Double Quotes

---

Still baffled - works in a javascript file but not a json file. Is there something special I have to do to get a . recognised as a trigger character in a json file (other than listing it in the vscode.languages.registerCompletionItemProvider as below)?

I have stripped it down to:

function activate(context) {

  loadLaunchSettings(context);
  activeContext = context;

  let docFilter = { scheme: 'file', language: 'json' };

  const configCompletionProvider = vscode.languages.registerCompletionItemProvider (
    // { language: 'json', pattern: './**/test.json' }, // does not work
    // 'javascript',                                    //  works
    // 'json',                                          // does not work
    docFilter,                                          // does not work
    {
       // eslint-disable-next-line no-unused-vars
      provideCompletionItems(document, position, token, context) {

        return [
          new vscode.CompletionItem('howdy1', vscode.CompletionItemKind.Text),
          new vscode.CompletionItem('howdy2', vscode.CompletionItemKind.Text),
          new vscode.CompletionItem('howdy3', vscode.CompletionItemKind.Text),
        ];
      }
    },
    "." // trigger
  );

    context.subscriptions.push(configCompletionProvider);
};