Chosen elements not responsive when handled by Drupal behavior

I dug in a bit to see what was going on, and I realize that I misconstrued the cause.

In fact what is going on with the current state of the module is that the javascript mixes percentage and pixel dimensions at line 158 and line 162 when use_relative_width is set:

        var dimension = this.settings.use_relative_width ? '%' : 'px';

        if ($element.width() < this.settings.minimum_width) {
          options.width = this.settings.minimum_width + dimension;
        }
        else {
          options.width = $element.width() + dimension;
        }

So what this means is that, when use_relative_width is set, the initial if compares the pixel width ($element.width()) with a percentage.

Given that 100% is usually the maximum percentage value that'll be set, and that 100px is a very small value for the size of a select, the if statement will almost always evaluate false (i.e. because the 100 entered as a percentage value is smaller than the pixel value of 223.9882 returned by jQuery).

This in turn means that we can wind up setting unhelpful values like width: 223.9882%.

The attached patch keeps percentage and pixel values in their own lanes :)

By the way, this may address some of the other issues here such as https://www.drupal.org/project/chosen/issues/2828214 (there are multiple similar-sounding issues filed against the 7.x branch as well).