Mason Only Add While Active (#496)

It's jarring for the example to automatically add to the button count,
so this makes it so it only increments while active is true.

---------

Co-authored-by: Daniel McNab <36049421+DJMcNab@users.noreply.github.com>

xilem/examples/mason.rs

@@ -92,6 +92,9 @@ fn app_logic(data: &mut AppData) -> impl WidgetView<AppData> {
             button("Reset", |data: &mut AppData| data.count = 0),
             flex((fizz_buzz_flex_sequence, flex_sequence)).direction(axis),
         )),
+        // The following `async_repeat` view only exists whilst the example is in the "active" state, so
+        // the updates it performs will only be running whilst we are in that state.
+        data.active.then(|| {
             async_repeat(
                 |proxy| async move {
                     let mut interval = time::interval(Duration::from_secs(1));
@@ -102,8 +105,11 @@ fn app_logic(data: &mut AppData) -> impl WidgetView<AppData> {
                         };
                     }
                 },
-            |data: &mut AppData, ()| data.count += 1,
-        ),
+                |data: &mut AppData, ()| {
+                    data.count += 1;
+                },
+            )
+        }),
     )
 }
 

xilem_core/src/views/fork.rs

@@ -115,9 +115,7 @@ struct NoElements;
 impl ElementSplice<NoElement> for NoElements {
     fn with_scratch<R>(&mut self, f: impl FnOnce(&mut AppendVec<NoElement>) -> R) -> R {
         let mut append_vec = AppendVec::default();
-        let ret = f(&mut append_vec);
-        debug_assert!(append_vec.into_inner().is_empty());
-        ret
+        f(&mut append_vec)
     }
 
     fn insert(&mut self, _: NoElement) {}