Created attachment 7336 Design glitch and all font relevant settings Xubuntu 17.10 beta 2 xfce4-notifyd 0.3.6-1 Look at screenshot... Maybe my font-settings are to large.
I can reproduce this also with the Gtk+ internal theme Adwaita, so it's definitely a problem related to the widget. The only property that controls the width is the width-chars property, which I would have to bump to 4 digits to make it work for your setup without visual truncation. This would look out of place in most other setups so I'm not sure there is a good solution to this. Unfortunately there is no way to tell the widget to grow along with the characters, at least as far as I can tell... Either way, this is a bug or lacking feature in the toolkit Gtk+, not in xfce4-notifyd (because it also affects all other applications that use spinbuttons).