viewport wrapping strings unnecessarily with UTF-8 characters

[cskeeters]

Describe the bug
When a string has a character with a value larger than 0x7F, it gets encoded into multiple bytes in UTF-8 encoding. When using the viewport component of bubbles, text that should fit within the width of one screen gets wrapped to the next line I suspect because it's counting bytes rather than Runes.

Setup
macOS Sonoma
ghostty or iterm2

locale:

LANG="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_CTYPE="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_ALL="en_US.UTF-8"

To Reproduce

1. Run wrap

   go mod wrap
   go build
   ./wrap

2. Change the width of the terminal. "B" should move to the next line only after the width of the terminal can't fit it on the same line.

3. Compare against behavior when string has no utf8 chars.

Source Code
main.go:

package main

import (
	"log"

	"github.com/charmbracelet/bubbles/viewport"
	tea "github.com/charmbracelet/bubbletea"
)

func main() {
	p := tea.NewProgram(initialModel(), tea.WithAltScreen())

	if _, err := p.Run(); err != nil {
		log.Fatal(err)
	}
}

type (
	errMsg error
)

type model struct {
	viewport    viewport.Model
	messages    []string
	err         error
}

func initialModel() model {
	vp := viewport.New(30, 5)

	// No UTF-8 Chars
	// vp.SetContent(`A                                                 B`)

	// UTF-8 Chars
	vp.SetContent(`A                                                 B`)

	return model{
		messages:    []string{},
		viewport:    vp,
		err:         nil,
	}
}

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

func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
	var (
		tiCmd tea.Cmd
		vpCmd tea.Cmd
	)

	m.viewport, vpCmd = m.viewport.Update(msg)

	switch msg := msg.(type) {
	case tea.WindowSizeMsg:
		m.viewport.Width = msg.Width
		m.viewport.Height = msg.Height

	case tea.KeyMsg:
		switch msg.Type {
		case tea.KeyCtrlC, tea.KeyEsc:
			return m, tea.Quit
		}

	// We handle errors just like any other message
	case errMsg:
		m.err = msg
		return m, nil
	}

	return m, tea.Batch(tiCmd, vpCmd)
}

func (m model) View() string {
	return m.viewport.View()
}

MY go.mod:

module wrap

go 1.24.0

require (
        github.com/charmbracelet/bubbles v0.20.0
        github.com/charmbracelet/bubbletea v1.3.4
)

require (
        github.com/aymanbagabas/go-osc52/v2 v2.0.1 // indirect
        github.com/charmbracelet/lipgloss v1.0.0 // indirect
        github.com/charmbracelet/x/ansi v0.8.0 // indirect
        github.com/charmbracelet/x/term v0.2.1 // indirect
        github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f // indirect
        github.com/lucasb-eyer/go-colorful v1.2.0 // indirect
        github.com/mattn/go-isatty v0.0.20 // indirect
        github.com/mattn/go-localereader v0.0.1 // indirect
        github.com/mattn/go-runewidth v0.0.16 // indirect
        github.com/muesli/ansi v0.0.0-20230316100256-276c6243b2f6 // indirect
        github.com/muesli/cancelreader v0.2.2 // indirect
        github.com/muesli/termenv v0.15.2 // indirect
        github.com/rivo/uniseg v0.4.7 // indirect
        golang.org/x/sync v0.11.0 // indirect
        golang.org/x/sys v0.30.0 // indirect
        golang.org/x/text v0.3.8 // indirect
)

Expected behavior
Would expect text to fit on one line when the terminal is wide enough.