To show indentation lines for nested markdown lists in CodeMirror, you can use the `indentWithTabs` and `tabSize` options in the CodeMirror configuration [4]. The `indentWithTabs` option specifies whether to use tabs for indentation or spaces, and the `tabSize` option specifies the number of spaces or tabs to use for indentation.

Here's an example configuration for CodeMirror that shows indentation lines for nested markdown lists:

var editor = CodeMirror.fromTextArea(document.getElementById("myTextArea"), {
  mode: "markdown",
  indentWithTabs: false,
  tabSize: 4,
  extraKeys: {"Enter": "newlineAndIndentContinueMarkdownList"}
});

In this example, the `indentWithTabs` option is set to `false` to use spaces for indentation, and the `tabSize` option is set to `4` to use four spaces for indentation. The `extraKeys` option is used to enable the `newlineAndIndentContinueMarkdownList` command, which automatically continues a markdown list when the Enter key is pressed.

Note that the `newlineAndIndentContinueMarkdownList` command only works for bullet lists, not for ordered lists [1]. To continue an ordered list, you need to manually type the next number or letter and a period after pressing Enter.

References:
- [1] https://github.com/codemirror/CodeMirror/issues/1276
- [2] https://codemirror.net/5/mode/markdown/
- [3] https://discuss.codemirror.net/t/nesting-ordered-and-bullet-lists-in-markdown-mode/3685
- [4] https://codemirror.net/5/doc/manual.html