Clean up shared UI runtime state

app-rust/tests/gesture_cancel.rs

@@ -87,7 +87,7 @@ fn a_press_ended_by_a_cancel_leaves_no_origin_for_the_next_one() {
 fn panning_a_code_fence_then_tapping_elsewhere_moves_nothing() {
     use ai_app::client::transcript_fold::{TranscriptItem, TranscriptRow};
 
-    let (mut h, screen) = opened();
+    let (mut h, mut screen) = opened();
     let fence = TranscriptRow::Single(TranscriptItem::AssistantMsg {
         seq: 9_000_000,
         text: "```\none two three four five six seven eight nine ten eleven twelve\n\