Fast Scroller works wrongly after customizing CCP Dialog theme.

Hello! I try to change the CCP Dialog size by these [two ways] (https://github.com/hbb20/CountryCodePickerProject/issues/265#issuecomment-458756357), and both of them are available, but they cause a bug too.
If the CCP Dialog size is changed and the height is not matching parent, the Fast Scroller will jump to the wrong position when user click or drag the scroller bar and cause the list jump to the wrong place.
By the way, if the CCP Dialog have the properties about size will make it better in my opinion.

Start by reproducing the issue using the two CCP Dialog resizing approaches linked in issue #265, with the dialog height smaller than its parent. Trace the Fast Scroller behavior when clicking and dragging its bar; done means both actions position the country list correctly after the dialog is resized.

Written by the indexing model from the issue text.