fix: adjust cursor start position for large buffers

[qilme]

nushell/nushell#15436

[fdncred]

Thanks for trying to make nushell and reedline better. Please help me understand how this fixes the issue. If my screen height is 50 how is queuing 50 \n's going to help? Is there a way we can easily try this in the reedline demo to see if it works?

[qilme]

Since reedline uses crossterm and crossterm uses ansi escape sequences to implement the function, I can use nushell to emulate the behavior.

1. scope commands or other command judged as large buffer.If it is a lagre buffer, then prompt_start_row is 0 and nushell send "ESC [0;0H" to terminal.

2. ansi -e "H" is the same as "ESC [0;0H". It clear my viewpoint in the Windows Terminal, vscode, alacritty and termux.

3. Scrool up and content as high as my screen height disappears.

4. scope commands

5. for x in 1..50 { if $x > 30 { break }; print ""}

6. ansi -e "H"

7. Scrool up and content exists

[fdncred]

But don't you have a bunch of empty lines after the buffer?

[qilme]

The demo does have, but the nushell I compiled after patching not.

[qilme]

Origin:

My demo:

My patch:

[Bahex]

I was experiencing this issue but I wasn't sure if it was caused by my terminal emulator or nushell/reedline. Thank you for working on this!

[Bahex]

Thanks!

[blindFS]

@qilme @Bahex Sorry to step in, I've noticed a panic caused by this change.

Basically we need saturating_sub to avoid negative numbers.
Besides, I'm not sure whether we should take lines.hint into account here.

[blindFS]

I've pushed what I consider to be a proper fix to #898 (since there might be a value useful for both issues), Please review that when you get time.

[Bahex]

@blindFS sorry for the slow reply, your PR was already merged by the time i got the time to take a look so i just forgot about it.

Besides, I'm not sure whether we should take lines.hint into account here.

getting a multiline hint can similarly push text up, not taking that into account can cause lines to disappear as well.

[blindFS]

getting a multiline hint can similarly push text up, not taking that into account can cause lines to disappear as well.

Say we have a large_buffer repaint caused by a long multiline hint, if we add the height of self.hint to lines.required_lines, we'll end up not doing any scrolling. Is that expected behavior?