The “Remember password” checkbox in the KDE su dialog is cropped. I think this was already the case with plasma 5.x on 13.2.
When you run into this thing which is obviously a coding issue, you should submit to https://bugzilla.opensuse.org
Note that the error you identified can be seen when a theme for visually impaired (very large print and icons) is applied which is not the case here.
TSU
This is only a problem with the breeze theme (it doesn’t happen with oxygen e.g.).
And no, it won’t help reporting such things to openSUSE, better report them to KDE upstream.
http://bugs.kde.org/
Thanks for pointing me in the right direction!