Created attachment 6862 Tiny scrollable popup The new version insists on using a tiny, scrollable popup, regardless of me setting a high number in history size. The previous version showed all entries at once.
I see that too since program was updated to use GTK+ 3. Menu's geometry is calculated once when it's created, but is not recalculated when new elements are added. To reproduce the issue, first start program with empty list (or clear list and restart). Click on icon to open clipboard contents menu. Then try to copy some more text. Then open menu again. It then will be scrollable.
Created attachment 6889 adjust menu geometry to fit all elements Proposed patch.
Nice catch, your patch indeed fixes the issue! I'll consider reverting it though whenever the Gtk3 panel is ready and released as this bug was not reproducible with the current WIP version (https://git.xfce.org/users/andrzejr/xfce4-panel/log/?h=gtk3).