When the GTK3 backend is active in current LibreOffice master (towards 6.1) some of the dialogs are now comprised of fully native GTK dialogs and widgetery. Instead of VCL widgetery themed to look like GTK, they're the real thing.
So for these dialogs this means f.e that the animated effects for radio and checkbuttons work, that the scrolling is overlay scrolling, and that the visual feedbacks that scrolling has reached its limit, or that available content is outside the scrolling region are shown. In the above demo, the GtkNotebook is the real thing.
I'm particularly pleased with the special character dialog because it has a custom character grid widget which comes with accessibility support which remains working when reworked as a native gtk widget with custom drawing and interaction callbacks.