> This is supposed to be the Emacs answer to the various IDEs being able
> to fold code, right?  Then let's try to make it look like in those IDEs.

This is an interesting question.  I tried to search how outlines look
in other IDEs, and found such a screenshot for VSCode.

IIUC, here the first column with a red circle for a breakpoint
corresponds to Emacs fringes, the second column with line numbers
is the same as display-line-numbers-mode, and the third column is
for outline arrows.

So the column for outline indicators could be implemented the same way
as display-line-numbers-mode.  Excuse my ignorance, is
display-line-numbers-mode implemented by using margins?