I haven't really looked into this, but is it possible to make GTK4 apps look liek standard GTK2/GTK3 applications? It feels like every single modern GTK app I've encountered has that modern Rounded-Material look to them and ignores the window manager decorations.
> and ignores the window manager decorations.
That's because Gtk4 does "client side decoration". That has the advantage (or otherwise, depending on your point of view!) that the application can now place custom widgets in the title bar, and the disadvantage that when apps do that, the part of the title bar available for dragging windows around becomes significantly smaller.
My main objection to client-side decoration is that middle-clicking a window's title bar to push it to the back no longer works. (Plus, for those of us with eyes that aren't as young as they once were, it's now much harder to choose a window border style that clearly indicates which window has focus.)