There are plenty of dialogs under Gtk3 which the OK/Cancle buttons are falling out of the screen area, due to the dialog too high/tall and can not be resizable.
This is a meta bug tracking all these dialogs.
This fault/bug is also on all my laptops using version 220.127.116.11 as at 25th April 2020 and has been present in all 6.4 versions.
As openSUSE Leap 15.1 uses their repository version, at present LibreOffice 18.104.22.168 and with XFCE all installed LibreOffice versions display this 'out of screen' dialogue error. There is no ability to resize for height of dialogue (width is resizeable). Can the smaller size with scroll bars to display full dialogue by scrolling be re-instated?
"Can the smaller size with scroll bars to display full dialogue by scrolling be re-instated?" I'm unaware that there was ever such scrollbars on the gtk dialogs.
As a workaround, though not ideal, it may be possible to use hold "super" (typically the windows logo button) and click on the dialog and drag to move it around to get the button area visible.
*** Bug 146675 has been marked as a duplicate of this bug. ***
*** Bug 151793 has been marked as a duplicate of this bug. ***