This seems to be a bug in GTK according to https://bugs.launchpad.net/ubuntu/+source/gtk2-engines-oxygen/+bug/1242801
(there a similar problem for Meld was reported).
Another workaround mentioned there is
For oxygen, edit the normally already existing file "/usr/share/themes/oxygen-gtk/gtk-2.0/gtkrc" and change
GtkComboBox::appears-as-list = 1
into
GtkComboBox::appears-as-list = 0
This seems to be a bug in GTK according to https:/ /bugs.launchpad .net/ubuntu/ +source/ gtk2-engines- oxygen/ +bug/1242801 themes/ oxygen- gtk/gtk- 2.0/gtkrc" and change ::appears- as-list = 1 ::appears- as-list = 0
(there a similar problem for Meld was reported).
Another workaround mentioned there is
For oxygen, edit the normally already existing file "/usr/share/
GtkComboBox
into
GtkComboBox