fix(viewport): prevent panic when height is negative or zero #880
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When viewport height becomes negative (e.g., during terminal resize), visibleLines() could panic with "slice bounds out of range" because bottom becomes less than top in the slice operation. Add a guard to return nil when top >= bottom to prevent the panic. Fixes charmbracelet#879 Co-Authored-By: Claude Opus 4.5 <[email protected]>
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Summary
visibleLines()to prevent slice bounds panic whentop >= bottomDetails
When viewport height becomes negative (e.g., during terminal resize with other components),
visibleLines()panics with"slice bounds out of range [N:M]" where N > M.
This happens because:
h = Height - Style.GetVerticalFrameSize()becomes negativebottom = clamp(YOffset + h, top, len(lines))becomes less thantoplines[top:bottom]panicsFixes #879