Outline tree structure when using a mix of headings

When the outline uses a mix of headings where some only have ##-headings without subheadings while others have ##-headings and ###-subheadings, then the tree structure menu is not shown as expected.

Steps to reproduce

# FooBar

## Foo
Observed: Shown without a DownArrow, slightly indented (as expected).

## Bar
Observed: Shown with a DownArrow, DownArrow increases the indent.
Expected: Indent is same level as any other ##-heading; DownArrow doesn't increase Indent.

### Bar 1
Observed: Indented to almost same level as Bar-parent (as expected).

### Bar 2

Expected result

When the outline uses a mix of headings where some only have ##-headings without subheadings while others have ##-headings and ###-subheadings, then DownArrow doesn’t affect how items are indented in the tree structure menu.

Actual result

When the outline uses a mix of headings where some only have ##-headings without subheadings while others have ##-headings and ###-subheadings, then the tree structure menu is not shown as expected.

Environment

Windows 10
Version: 1.1.9

Additional information