Same problem here, then I solved it by moving non-heading lines before the first heading.
Basics
(For anyone like me cannot find the outline button)
If you search “Table of content”, “Outline”, “sidebar” you should refer to this section.
On Mac, use CMD+, to open the preference window quickly.
Search “outline” in core plugins and toggle it to enable outline.
steps to solve the problem
As @argentum mentioned, the first line of the document must be a level 1 head, or start in the form of “# xxxx”.
Before modification
The variable and functions have to be handled separately. (first line of docu.)
# Basic conception
A device variable is a symbol seen by the host.
After modification
# Basic conception (first line of docu.)
The variable and functions have to be handled separately.
A device variable is a symbol seen by the host.
After modification, the outline is still empty.
Disabling then enabling “outline” does not solve the problem.
Closing the document then reopening it solved the problem.