Viewport ScrollPercent() sometimes returns `NaN`

if `y.Offset` = 0 and the terminal window size is **almost large enough** to contain all of the content in the viewport, `ScrollPercent()` will return `NaN`.

If the `m.Height >= len(m.lines)` your already returning 1.0 which works great, maybe it should be len(m.lines - 1) because you also have `t := float64(len(m.lines) - 1)`? Not sure yet, I haven't debugged it yet, but you can see it here in the default bubbletea/examples/pager`.