fix(ui): prevent wheel lockup at edge in non-continuous view (#5713)

crates/ui-egui/src/canvas/navigation.rs

@@ -268,7 +268,11 @@ impl DocView {
         let Some(forward) = self.wheel.feed(unit, dy, phase, now, can_turn) else { return false };
         let before = self.current;
         self.step_page(forward);
-        self.current != before
+        if self.current == before {
+            self.wheel.unturn();
+            return false;
+        }
+        true
     }
 
     /// One wheel event in a non-continuous view whose page is taller than the window, where the
@@ -288,6 +292,7 @@ impl DocView {
         let before = self.current;
         self.step_page(forward);
         if self.current == before {
+            self.wheel.unturn();
             return false;
         }
         if !forward {

crates/ui-egui/src/canvas/wheel_pager.rs

@@ -73,6 +73,11 @@ impl WheelPager {
         self.accum = 0.0;
     }
 
+    /// Reset any turned state when a turn did not happen (e.g. at document boundary).
+    pub(crate) fn unturn(&mut self) {
+        self.turned = None;
+    }
+
     /// Whether the current gesture already turned a page: the rest of it must not pan the page
     /// just shown away from its top (or bottom).
     pub(crate) fn gesture_turned(&self) -> bool {

crates/ui-egui/tests/it/view.rs

@@ -520,6 +520,48 @@ fn single_page_swipe_that_reaches_the_edge_does_not_turn_but_a_fresh_one_does()
     );
 }
 
+#[test]
+fn single_page_wheel_at_last_page_bottom_edge_can_scroll_back_up() {
+    let mut h = harness_stepping(1.0 / 60.0, &[("layout", "single"), ("zoom", "200")]);
+    h.state_mut().views[0].go_to_page(2); // Go to last page of PAGES (3 pages)
+    h.run_steps(20);
+    let at = h.state().views[0].viewport_rect().center();
+    h.hover_at(at);
+    h.run_steps(4);
+
+    // Scroll to the bottom edge of page 3
+    let mut notches = 0;
+    let mut last = rect(&h, 2).expect("page 3 on screen").min.y;
+    let mut at_bottom = false;
+    while notches < 80 {
+        h.event(wheel(MouseWheelUnit::Line, -1.0, TouchPhase::Move));
+        h.run_steps(12);
+        notches += 1;
+        if let Some(r) = rect(&h, 2) {
+            if (r.min.y - last).abs() < 0.01 {
+                at_bottom = true;
+                break;
+            }
+            last = r.min.y;
+        }
+    }
+    assert!(at_bottom, "reached bottom edge of last page");
+    assert_eq!(h.state().views[0].current, 2, "remains on page 3");
+
+    // Push the wheel further against the bottom edge (cannot turn because it's the last page)
+    for _ in 0..5 {
+        h.event(wheel(MouseWheelUnit::Line, -1.0, TouchPhase::Move));
+        h.run_steps(12);
+    }
+    let bottom_y = rect(&h, 2).expect("page 3 on screen").min.y;
+
+    // Roll the wheel back up: the page must move away from the bottom edge (#5713)
+    h.event(wheel(MouseWheelUnit::Line, 1.0, TouchPhase::Move));
+    h.run_steps(12);
+    let after_up_y = rect(&h, 2).expect("page 3 on screen").min.y;
+    assert!(after_up_y > bottom_y, "scrolling up moves the page away from the bottom edge: {after_up_y} > {bottom_y}");
+}
+
 #[test]
 fn stepping_past_either_end_leaves_the_view_alone() {
     // Paging on past the first or last page must not jump the page back to its top.