bug#56820: outline-minor-mode replacing the first character with an arrow

[Juri Linkov]

>> > Vdisplay_line_numbers is used in xdisp.c for several purposes:
>> >
>> >   . adjust layout calculations for the space taken by line numbers
>> >   . disable some redisplay optimizations that cannot be used when line
>> >     numbers (especially relative numbers) are displayed
>> >   . produce and display the line numbers for relevant lines
>> >
>> > Which one(s) are "strategy" for the purpose of this discussion?
>> 
>> The strategy is to reuse the existing code that adjusts layout calculations
>> to display the line numbers, and to display outline arrows after line 
>> numbers.
>
> So you are talking only about the first bullet above.

Sharing the existing code for the first bullet indeed.
Then implementing the display of images for relevant outlines.