Viewport's maxYOffset() should consider top / bottom borders

[Andrew Albers]

When a border style is applied to a viewport, the last line (or two lines) of the viewport's rendered content will be hidden when scrolling or calling SetYOffset(). This is because setting YOffset is clamped to maxYOffset(), which doesn't account for the fact that a border subtracts from its content-renderable height.

Ex. Identical content rendered in un-bordered & bordered viewports after scrolling all the way down
(note that line 9 and line 10 cannot be reached):

Example code

package main

import (
	"fmt"
	"os"

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

const (
	viewH   = 5
	viewW   = 10
	content = "line 1\nline 2\nline 3\nline 4\nline 5\nline 6\nline 7\nline 8\nline 9\nline 10"
)

var (
	borderStyle   = lipgloss.NewStyle().Border(lipgloss.RoundedBorder())
	noBorderStyle = lipgloss.NewStyle()
)

type model struct {
	view1 viewport.Model
	view2 viewport.Model
}

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

func (m *model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
	m.view1, _ = m.view1.Update(msg)
	m.view2, _ = m.view2.Update(msg)
	return m, nil
}

func (m *model) View() string {
	view1Content := m.view1.View()
	view2Content := m.view2.View()
	combined := lipgloss.JoinHorizontal(lipgloss.Left, view1Content, view2Content)
	return combined
}

func main() {
	view1 := viewport.New(viewW, viewH)
	view1.Style = noBorderStyle
	view1.SetContent(content)

	view2 := viewport.New(viewW, viewH)
	view2.Style = borderStyle
	view2.SetContent(content)

	m := &model{
		view1: view1,
		view2: view2,
	}

	p := tea.NewProgram(m)
	if _, err := p.Run(); err != nil {
		fmt.Println("Run error:", err)
		os.Exit(1)
	}
}

[Andrew Albers]

Created a PR with my suggested fix: #337