fix: cursor position adjustment after exiting alt screen

[semihbkgr]

When exiting alternate screen mode, the cursor was not properly adjusted, which caused terminal content to be either overwritten (if the current line count was greater than before entering alt screen) or leftover lines to remain at the top (if the current line count was smaller). To fix this, we save the number of lines rendered before entering alternate screen mode. When exiting, we compare the current line count with the saved value and adjust the cursor position accordingly, ensuring correct positioning and preventing both overwriting and leftover lines.

fixes #1013

[semihbkgr]

In my understanding, the source of this issue appears to be that the terminal automatically moves the cursor back to its position, where it was before entering alt screen mode, when exiting.

[meowgorithm]

Thanks for this, @semihbkgr. Can you also paste in some code for a minimally reproducible example? It would also be good to know if this is a regression in v1.2.0.

[semihbkgr]

Hi @meowgorithm

Can you also paste in some code for a minimally reproducible example?

There is already an example in the issue mentioned in the description. It was a real case we encountered in kubectl-klock. However, I will share a simplified example to reproduce the bug more easily, which directly highlights the problematic situation 👍

It would also be good to know if this is a regression in v1.2.0.

The issue dates back to older versions. The issue was created on May 10, indicating it was first observed in v0.26.2 or earlier.

[semihbkgr]

Operations

• j: increments line count
• k: decrements line count
• <space>: toggle alt screen

The issue occurs when the number of lines rendered in alt screen mode differs from the lines rendered before entering alt screen mode — either fewer lines (leaving leftover lines) or more lines (overwriting old lines).

Reproducing the Overwriting Old Lines Bug:

1. Start the program and render some initial content in the terminal (e.g., lines 1 to 5).
2. Press <space> to toggle into alt screen mode.
3. Press j multiple times to increment the line count and render additional lines in the alt screen.
4. Press <space> again to exit alt screen mode.
5. Observe that the additional lines rendered in the alt screen overwrite the existing terminal content (lines 1 to 5).

Reproducing the Keeping Leftover Lines Bug:

1. Start the program and render some initial content in the terminal (e.g., lines 1 to 5).
2. Press <space> to toggle into alt screen mode.
3. Press k multiple times to decrement the line count and reduce the number of rendered lines.
4. Press <space> again to exit alt screen mode.
5. Observe that the leftover lines from the initial content remain visible above the current lines rendered in the alt screen.

package main

import (
	"fmt"
	"os"
	"strings"

	tea "github.com/charmbracelet/bubbletea"
)

type model struct {
	altscreen bool
	n         int
}

func (m model) Init() tea.Cmd {
	return nil
}

func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
	switch msg := msg.(type) {
	case tea.KeyMsg:
		switch msg.String() {
		case "q", "ctrl+c", "esc":
			return m, tea.Quit
		case "j":
			m.n += 1
		case "k":
			m.n -= 1
		case " ":
			var cmd tea.Cmd
			if m.altscreen {
				cmd = tea.ExitAltScreen
			} else {
				cmd = tea.EnterAltScreen
			}
			m.altscreen = !m.altscreen
			return m, cmd
		}
	}
	return m, nil
}

func (m model) View() string {
	b := strings.Builder{}
	if m.altscreen {
		b.WriteString("alt screen\n")
	} else {
		b.WriteString("normal screen\n")
	}
	for i := 0; i < m.n; i++ {
		b.WriteString(fmt.Sprintf("%d. line\n", i))
	}
	return b.String()
}

func main() {
	if _, err := tea.NewProgram(model{}).Run(); err != nil {
		fmt.Println("Error running program:", err)
		os.Exit(1)
	}
}

[meowgorithm]

Thanks for that; we're taking a look.

[aymanbagabas]

LGTM, thank you, @semihbkgr!

[meowgorithm]

Awesome fix, @semihbkgr: thank you. We'll get a patch release out today.

[meowgorithm]

Alright, v1.2.3, containing the fix, is available now. Thanks for the fix @semihbkgr!

[unitsvc]

clear screen , then tea.EnterAltScreen -> tea.ExitAltScreen, sometimes it does not work on mac or linux systems, the screen cannot exit full screen, or flickers.
@semihbkgr @meowgorithm