fix: ensure the parent of $testInput exists within the body

[yfxie]

In some scenarios of using SPA, measureString can correctly calculate the width of $testInput on the first call. However, due to various possibilities, changes to the DOM will cause $testInput to leave the body, and the calculation result of measureString will always be 0. This PR ensures that $testInput always exists under the body to calculate the correct width.

The following is a rough example of reproducing the bug:

// Append $testInput to the body
$("input").appendTo("body").selectize(); 

// $testInput leaves the body
$("body").html(""); 

// measureString gets incorrect results
$("input").appendTo("body").selectize(); 

[github-actions[bot]]

This pull request is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days

[yfxie]

@risadams can help review this PR?

[github-actions[bot]]

This pull request is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days