With the change:
The gtk version to be used by the JDK was fixed to 2.2. Checking on a recent Ubuntu Version (21.04, Hirsute Hippo) with Gnome and Yaru-light theme, the issues reported in
NETBEANS-1344 seems to be fixed with recent JDKs (Java: 11.0.11; OpenJDK 64-Bit Server VM 11.0.11+9-Ubuntu-0ubuntu2).
Screenshots of the toolbars (first is normal size, second is small size):
Output window with selection and toolbars:
List selection highlight:
Document tabs with colored groups:
While not my preferred LAF, to me the gtk2 version looks worse, than the gtk3 version.