Trouble with vterm-clear affecting buffer history

[ashlineldridge]

First, thanks for the great package.

I'm having trouble with vterm-clear destroying buffer history in strange ways. I currently have vterm-clear-scrollback-when-clearing set to nil, so vterm-clear is not calling vterm-clear-scrollback directly. However, the odd behavior I'm seeing can be described in two scenarios:

1. If I open a new vterm buffer and type some commands whose output does not cause the buffer to scroll (e.g., pwd), and then type either C-l, M-x vterm-clear, or M-x vterm-send-C-l, the buffer is cleared and scrollback is cleared. I'm effectively seeing the same behavior as if I had run C-c C-l.

2. If I open a new vterm buffer and type some commands whose output does cause the buffer to scroll (e.g., find /), and then type either C-l, M-x vterm-clear, or M-x vterm-send-C-l, the buffer is cleared as expected but scrollback is not cleared and I can move point up to see previous lines. However, at this point if I type subsequent commands which do not cause the buffer to scroll (e.g., pwd) and then again type C-l, the history of my last (short output) command is lost. When I scroll up I only see the original command and its output which caused the buffer to scroll (in my example, find /).

Thanks for any assistance.

OS: macOS 10.15.7 Emacs: 27.2 Emacs-libvterm: sha 2681120 (current origin/master HEAD) libvterm: I let the libvterm package install its own version

[Dieken]

got this issue too. /usr/bin/clear or tput clear sends <ESC>[H<ESC>[2J to vterm:

<ESC>[H: move to position (1,1) <ESC>[2J: clear all screen, not include saved lines and alternative screen.

Although vterm-clear-scrollback-when-clearing is nil, but the text content in current screen before clear isn't saved to scrollback buffer, so you can scroll back to previous screen, but can't scroll back to latest output:

before clear:

xxxxx   <- on previous screen, in scroll buffer
-----> begin of current screen
yyyyy  <- on current screen
$ tput clear <- emit command


<----end of current screen

after clear, "yyyyy" is missing, this isn't consistent with the behaviour of other terminal emulator.

xxxxx   <- on previous screen, STILL in scroll buffer
-----> begin of current screen
$          <- prompt here, but you can't scroll back to see "yyyyy" any more.

<----end of current screen

[ashlineldridge]

@Dieken Glad it's not just me then. Because libvterm is running as an (often small) Emacs window, I find that I press C-l a lot more than I would in a dedicated terminal application, so this issue affects me more than it would otherwise. I can't really rely on libvterm history at this point - whether the command/output is present in the history depends on whether it caused the buffer to scroll or not.

[mpolden]

I'm also seeing this behavior on macOS, with default vterm configuration. C-l / vterm-clear always clears scrollback.

[matteo-l]

I have the same behaviour with the following setup

OS: Ubuntu 20.04 Emacs: 28.2 Emacs-libvterm: sha 94e2b0b libvterm: 0.1.2-2