UiContext::note_scroll_event: the wheel-gesture bookkeeping as a call hosts can make

propagate_event's new-gesture detection (a 250ms gap clears the
initiator) was reachable only through propagate_event. A host that hands
a wheel straight to a widget's handle_event — the designer's modal
dialog dispatching its own panes — left the flags as the last routed
wheel set them, so a fresh gesture over the dialog's Settings list read
as the tail of one the MAIN params pane owned, the ownership rule never
engaged, and the control under the pointer took every event (a finger
scroll turned Grid Thickness from 30 to 26). Factored out; the dialog
calls it.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

 src/context.rs | 57 ++++++++++++++++++++++++++++++++++++---------------------
 1 file changed, 36 insertions(+), 21 deletions(-)

diff --git a/src/context.rs b/src/context.rs
index 1fb271d..f5d6578 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -136,33 +136,48 @@ impl UiContext {
     /// name roots by id and the registry is the one place a pointer lives). The root must be
     /// registered — apps already register every widget for focus/coverage — and an
     /// unresolvable root is a loud no-op, never a deref.
+    /// The scroll-gesture bookkeeping every wheel dispatch must pass
+    /// through: a gap over 250ms since the last wheel starts a NEW gesture
+    /// (`scroll_gesture_new`, and the initiator is cleared), a shorter gap
+    /// continues the current one. `propagate_event` calls this for the
+    /// wheels it routes; a host that hands a wheel straight to a widget's
+    /// `handle_event` (the designer's modal dialog, whose panes it dispatches
+    /// itself) calls it first — or the flags stay whatever the last routed
+    /// wheel left, and a pane inside the dialog reads a fresh gesture as the
+    /// tail of one the MAIN pane owned and lets the control under the pointer
+    /// take it (2026-09-20: the Settings list would not scroll after the
+    /// params pane had).
+    pub fn note_scroll_event(&mut self) {
+        let now = std::time::Instant::now();
+        let elapsed_ms = match self.last_scroll_time {
+            None => 999999,
+            Some(last) => now.duration_since(last).as_millis(),
+        };
+        if elapsed_ms >= 5 {
+            let is_new_gesture = elapsed_ms > 250;
+            if is_new_gesture {
+                self.scroll_initiate_widget_id = None;
+                self.scroll_gesture_new = true;
+            } else {
+                self.scroll_gesture_new = false;
+            }
+            if crate::scroll_debug() {
+                eprintln!(
+                    "[scroll] router: gap={elapsed_ms}ms new_gesture={is_new_gesture} initiator={:?}",
+                    self.scroll_initiate_widget_id
+                );
+            }
+            self.last_scroll_time = Some(now);
+        }
+    }
+
     pub fn propagate_event(&mut self, event: &Event, root: WidgetId) -> bool {
         let Some(root_ptr) = self.tree.get_ptr(root) else {
             eprintln!("propagate_event: unregistered/stale root {root:?} — event dropped");
             return false;
         };
         if let Event::MouseWheel { .. } = event {
-            let now = std::time::Instant::now();
-            let elapsed_ms = match self.last_scroll_time {
-                None => 999999,
-                Some(last) => now.duration_since(last).as_millis(),
-            };
-            if elapsed_ms >= 5 {
-                let is_new_gesture = elapsed_ms > 250;
-                if is_new_gesture {
-                    self.scroll_initiate_widget_id = None;
-                    self.scroll_gesture_new = true;
-                } else {
-                    self.scroll_gesture_new = false;
-                }
-                if crate::scroll_debug() {
-                    eprintln!(
-                        "[scroll] router: gap={elapsed_ms}ms new_gesture={is_new_gesture} initiator={:?}",
-                        self.scroll_initiate_widget_id
-                    );
-                }
-                self.last_scroll_time = Some(now);
-            }
+            self.note_scroll_event();
         }
         if let Event::KeyInput(ref key_event) = event {
             let is_scroll_key = match &key_event.logical_key {