If you are running Dia from the nigthly GNOME flatpak, the last update brought a big change: it is now a GTK 3 app. The UI shouldn't be much different, just some cosmetic adjustments due to theming and other like widget spacing. Also it now work with Wayland and HiDPI.
It is something I started over two years ago, and that for some reason I left on the back burner. So I did a final push and many thanks to Zander for reviewing and merging it. Now we shall hunt for regression. I already have a fix for the touchpad scrolling.
The version is numbered starting 0.98. So if it is Dia 0.98 or higher, then it's GTK 3.
There is still some more stuff to do. Fix a regression in the navigator widget, build without deprecation post GTK 3.8, etc.