shell: implement minimize

src/backend/kms/mod.rs

@@ -1068,7 +1068,12 @@ fn render_node_for_output(
     let nodes = workspace
         .get_fullscreen()
         .map(|w| vec![w.clone()])
-        .unwrap_or_else(|| workspace.windows().collect::<Vec<_>>())
+        .unwrap_or_else(|| {
+            workspace
+                .mapped()
+                .map(|mapped| mapped.active_window())
+                .collect::<Vec<_>>()
+        })
         .into_iter()
         .flat_map(|w| w.wl_surface().and_then(|s| source_node_for_surface(&s)))
         .collect::<Vec<_>>();