Clean up shared UI runtime state

app-rust/tests/fence_fling.rs

@@ -49,7 +49,7 @@ fn a_flick_across_a_code_fence_keeps_moving_after_the_finger_leaves() {
 
     let mut h = Harness::new(phone_size(), PHONE_SCALE);
     let opened = ai_app::ui::fixture::open(&mut h.rsc, &mut h.state).expect("the fixture folds");
-    let screen = opened.screen;
+    let mut screen = opened.screen;
     h.frame(0);
     h.frame(PHONE_FRAME_MS);
 
@@ -119,7 +119,7 @@ fn a_drag_away_from_a_coasting_fence_scrolls_the_list_and_leaves_it_coasting() {
 
     let mut h = Harness::new(phone_size(), PHONE_SCALE);
     let opened = ai_app::ui::fixture::open(&mut h.rsc, &mut h.state).expect("the fixture folds");
-    let screen = opened.screen;
+    let mut screen = opened.screen;
     h.frame(0);
     h.frame(PHONE_FRAME_MS);