Fix mount use after removal/Simplify mounts a bit (#2834)

* remove clone mounted; mounted nodes should never leak into components?

* Remove the mount from vnodes after unmounting them

* Fix use after removal in iterator diffing

* remove logs

* Re-fix keyed list diffing

* Use new.len for diffing keyed lens and remove redundant check

* simplify core list diffing logic a bit

* remove println

* add proper asserts for new tests

---------

Co-authored-by: Jonathan Kelley <jkelleyrtp@gmail.com>

packages/core/tests/diff_dynamic_node.rs

@@ -47,3 +47,68 @@ fn toggle_option_text() {
         ]
     );
 }
+
+// Regression test for https://github.com/DioxusLabs/dioxus/issues/2815
+#[test]
+fn toggle_template() {
+    fn app() -> Element {
+        rsx!(
+            Comp {
+                if true {
+                    "{true}"
+                }
+            }
+        )
+    }
+
+    #[component]
+    fn Comp(children: Element) -> Element {
+        let show = generation() % 2 == 0;
+
+        rsx! {
+            if show {
+                {children}
+            }
+        }
+    }
+
+    let mut dom = VirtualDom::new(app);
+    dom.rebuild(&mut dioxus_core::NoOpMutations);
+
+    // Rendering again should replace the placeholder with an text node
+    dom.mark_dirty(ScopeId::APP);
+    assert_eq!(
+        dom.render_immediate_to_vec().edits,
+        [
+            CreatePlaceholder { id: ElementId(2) },
+            ReplaceWith { id: ElementId(1), m: 1 },
+        ]
+    );
+
+    dom.mark_dirty(ScopeId(ScopeId::APP.0 + 1));
+    assert_eq!(
+        dom.render_immediate_to_vec().edits,
+        [
+            CreateTextNode { value: "true".to_string(), id: ElementId(1) },
+            ReplaceWith { id: ElementId(2), m: 1 },
+        ]
+    );
+
+    dom.mark_dirty(ScopeId(ScopeId::APP.0 + 1));
+    assert_eq!(
+        dom.render_immediate_to_vec().edits,
+        [
+            CreatePlaceholder { id: ElementId(2) },
+            ReplaceWith { id: ElementId(1), m: 1 },
+        ]
+    );
+
+    dom.mark_dirty(ScopeId(ScopeId::APP.0 + 1));
+    assert_eq!(
+        dom.render_immediate_to_vec().edits,
+        [
+            CreateTextNode { value: "true".to_string(), id: ElementId(1) },
+            ReplaceWith { id: ElementId(2), m: 1 },
+        ]
+    );
+}