Steps to reproduce the issue

Check the field type 'folderlist' in a generic extension parameters:

Expected result

All dropdown values are the names of the folders as always been

Actual result

All dropdown values includes the full path to the folders on Windows

System information (as much as possible)

Joomla 3.6 and later is affected, due to changes in the file root/libraries/joomla/form/fields/folderlist.php

The line 189:
$path = JPATH_ROOT . '/' . $path;

works only for Unix paths, fails on Windows. It should be:

$path = JPATH_ROOT . '\\' . $path;

but should be fixed as:

$path = JPATH_ROOT . DIRECTORY_SEPARATOR . $path;

Additional comments