How does kitty tell the position of previous command line prompts?

[xuhdev]

I'm playing with --stdin-source <STDIN_SOURCE>. However, @first_cmd_output_on_screen often got the output wrong, sometimes giving empty results. I haven't figured out the commonality among the scenarios that it fails.

Is there a document on how kitty determines the position of previous command line prompts? Perhaps I just don't understand how @first_cmd_output_on_screen works.

[kovidgoyal]

https://sw.kovidgoyal.net/kitty/shell-integration/#notes-for-shell-developers