Different behaviors for CSI 2 J at a prompt versus not

[iceghost]

At a shell, you can typically run ctrll to clear the screen and keep the scrollback intact. clear doesn't.

I tried to alias clear='printf "\x1b[H\x1b[2J"' to get the same behavior of ctrll, but apparently it didn't work like I expected: scrollback got clobbered.

My question being: why is there a difference?

---

Some more context: I consulted with @rockorager. Apparently we have the following snippet:

ghostty/src/termio/Termio.zig

Lines 475 to 505 in 8f5f432

	// If we're not at a prompt, we just delete above the cursor.
	if (!self.terminal.cursorIsAtPrompt()) {
	if (self.terminal.screen.cursor.y > 0) {
	self.terminal.screen.eraseRows(
	.{ .active = .{ .y = 0 } },
	.{ .active = .{ .y = self.terminal.screen.cursor.y - 1 } },
	);
	}
	// Clear all Kitty graphics state for this screen. This copies
	// Kitty's behavior when Cmd+K deletes all Kitty graphics. I
	// didn't spend time researching whether it only deletes Kitty
	// graphics that are placed baove the cursor or if it deletes
	// all of them. We delete all of them for now but if this behavior
	// isn't fully correct we should fix this later.
	self.terminal.screen.kitty_images.delete(
	self.terminal.screen.alloc,
	&self.terminal,
	.{ .all = true },
	);
	return;
	}
	// At a prompt, we want to first fully clear the screen, and then after
	// send a FF (0x0C) to the shell so that it can repaint the screen.
	// Mark the current row as a not a prompt so we can properly
	// clear the full screen in the next eraseDisplay call.
	self.terminal.markSemanticPrompt(.command);
	assert(!self.terminal.cursorIsAtPrompt());
	self.terminal.eraseDisplay(.complete, false);

So there is a difference of CSI 2 J whether we are running at a prompt or not:

• At a prompt: does a scrollClear.
• Not at a prompt: deletes rows.

So he asked me to open discussion to see if the difference is intentional.

[tml-adobe]

Some relevant discussion in xtermjs/xterm.js#1727