Send tiled state when prefer-no-csd is set

resources/default-config.kdl

@@ -91,6 +91,7 @@ cursor {
 
 // Uncomment this line to ask the clients to omit their client-side decorations if possible.
 // If the client will specifically ask for CSD, the request will be honored.
+// Additionally, clients will be informed that they are tiled, removing some rounded corners.
 // prefer-no-csd
 
 // You can customize the widths that "switch-preset-column-width" (Mod+R) toggles between.

src/handlers/xdg_shell.rs

@@ -31,6 +31,18 @@ impl XdgShellHandler for State {
             ws.configure_new_window(&window);
         }
 
+        // If the user prefers no CSD, it's a reasonable assumption that they would prefer to get
+        // rid of the various client-side rounded corners also by using the tiled state.
+        let config = self.niri.config.borrow();
+        if config.prefer_no_csd {
+            window.toplevel().with_pending_state(|state| {
+                state.states.set(xdg_toplevel::State::TiledLeft);
+                state.states.set(xdg_toplevel::State::TiledRight);
+                state.states.set(xdg_toplevel::State::TiledTop);
+                state.states.set(xdg_toplevel::State::TiledBottom);
+            });
+        }
+
         // At the moment of creation, xdg toplevels must have no buffer.
         let existing = self.niri.unmapped_windows.insert(wl_surface, window);
         assert!(existing.is_none());