viewport: panic with slice bounds out of range when height is negative or zero

[semihbkgr]

Description

The viewport panics with slice bounds out of range when the Height is set to a negative or zero value. This commonly occurs when resizing the terminal window to be very small while using a viewport alongside other components (e.g., textarea).

Stack Trace

runtime error: slice bounds out of range [8:4]

goroutine 1 [running]:
github.com/charmbracelet/bubbles/viewport.Model.visibleLines(...)
        viewport/viewport.go:150
github.com/charmbracelet/bubbles/viewport.(*Model).GotoBottom(...)
        viewport/viewport.go:348

Root Cause

In visibleLines() (viewport.go:143-150), when Height becomes negative:

func (m Model) visibleLines() (lines []string) {
    h := m.Height - m.Style.GetVerticalFrameSize()  // h can be negative
    // ...
    if len(m.lines) > 0 {
        top := max(0, m.YOffset)
        bottom := clamp(m.YOffset+h, top, len(m.lines))  // bottom can be < top
        lines = m.lines[top:bottom]  // PANIC: invalid slice bounds
    }
}

When h is negative and YOffset is positive (from a previous scroll), bottom becomes less than top, causing the panic.

Steps to Reproduce

1. Create a viewport with another component that takes up vertical space
2. Calculate viewport height as: msg.Height - otherComponent.Height() - gap
3. Shrink the terminal window until the calculated height becomes negative
4. The viewport panics

Minimal Example

case tea.WindowSizeMsg:
    m.viewport.Height = msg.Height - m.textarea.Height() - 2  // Can go negative
    m.viewport.GotoBottom()  // Triggers panic

Suggested Fix

Add a guard in visibleLines() to handle the case where top >= bottom:

if len(m.lines) > 0 {
    top := max(0, m.YOffset)
    bottom := clamp(m.YOffset+h, top, len(m.lines))
    if top >= bottom {
        return nil
    }
    lines = m.lines[top:bottom]
}