Running clear in terminal removes viewport content from buffer instead of hiding it

[Tyriar]

Steps to reproduce:

1. Run ll twice in to fill the viewport
2. Run clear
3. Scroll up and notice that a number of rows equal to the height of the terminal were removed from the buffer.

[Tyriar]

gnome-terminal appears to be buggy in this regard, it removes that many rows from the very beginning of the buffer, not the old viewport.

Here is the spec for the signal that's received (2):

ED – Erase In Display

ESC [ Ps J  default value: 0
This sequence erases some or all of the characters in the display according to the parameter. Any complete line erased by this sequence will return that line to single width mode. Editor Function

Parameter   Parameter Meaning
0   Erase from the active position to the end of the screen, inclusive (default)
1   Erase from start of the screen to the active position, inclusive
2   Erase all of the display – all lines are erased, changed to single-width, and the cursor does not move.

http://www.vt100.net/docs/vt100-ug/chapter3.html

The right thing might be to clear absolutely everything like xterm does.

[parisk]

So should we move forward with closing this issue and leave the cleaning behavior as is?

[Tyriar]

@parisk well it looks like a bug to me, maybe just doing what xterm does would be better considering that is what this is trying to emulate?

[parisk]

Well, did some tests. I think that your proposal of "shifting rows up" instead of removing them from buffer provides the best UX, even if it is not 100% xterm compliant.

[CoenraadS]

I just ran into this with: microsoft/vscode#19934

I would like to paste my use-case here for consideration:

My use-case (also how powershell works):

I like to cls and then run npm test. Then the beginning of my terminal is where my test output starts.
Now I cls, but I have no idea where the beginning of my test output is.

Therefor I disagree with just shifting rows up.

[Tyriar]

Here's a roundup of behavior in various terminal emulators when they receive an Erase All ED signal (\e[2J):

• xterm.js@2.3: keep everything, just push it up
• gnome-terminal: clear everything on top of the current viewport, then push the current viewport up
• Fedora terminal: same as gnome-terminal
• xterm: Clear entire buffer
• Terminal.app: same as xterm.js@2.3
• iTerm2 v3: same as xterm.js@2.3

Based on some references I found, it seems ambiguous what exactly this escape sequence is meant to be since this one says "Erase All", and this one says "Erase Screen". The actual xterm behavior however would seem to indicate that it means clear everything.

To me xterm's behavior seems the most reasonable, particularly when we implement search (#553), but I can see why the current behavior may be good for some users. @parisk what do you think? Should this be a setting (clear everything vs push up) or should we move to xterm's behavior?

[parisk]

Based on the name of the signal, I think as well that xterm's implementation is the most reasonable.

But, I think also that xterm.js' implementation is the one that you will regret less, if it's done accidentally 😅 , so for me (personally) it's more desirable.

What is the use case that one person would prefer the entire buffer getting erased to hidden?