Resizing the terminal while in the alt buffer will break the normal buffer

[Tyriar]

Downstream issue microsoft/vscode#19385

Steps to reproduce

• Have some output in the terminal like from ls -l
• Open vim
• :q
• You can still see the output from ls, good
• Open vim again
• Resize the terminal
• :q
• The vim text is still on screen, and I can't scroll to see the output from ls.

[Tyriar]

This is not as trivial as it looks. It's happening because the normal buffer is cleared in resize, this is done because without doing this it doesn't get resized which causes a lot of issues if the rows were reduced.

[mofux]

I noticed this when I was looking through the code the other day. When the alt screen buffer is created, the normal screen buffer is backed up to terminal.normal, but terminal.normal never seems to get modified, not even on resize. When we switch back to the normal buffer, the terminal.normal backup is restored.

InputHandler.ts - creating the alternate buffer
InputHandler.ts - restoring the normal buffer

But still, given the way the alt buffer is replaced by normal buffer, how can there still be any content from the alt buffer making its way back to the normal buffer? I assume this happens because we do not call terminal.reset() before restoring the normal buffer (notice terminal.reset() is called when the alt screen buffer is created). Is there a reason why terminal.reset() is not called?

[Tyriar]

normal = null means that the alt buffer becomes the normal buffer, so nothing happens when you switch back to the normal buffer after exiting vim. We probably want to call resize again after the normal buffer is switched in, reworking resize so it doesn't break when we call it again.