M13.17: the viewer keeps its pan after a middle-button or Hand drag — egui reports no total drag on the release frame, so the pan was recomputed as the drag's start and snapped back

crates/ui-egui/src/panels/viewer.rs

@@ -1097,8 +1097,11 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
         let merge = format!("viewer-drag-{}", ui.data(|d| d.get_temp::<u64>(egui::Id::new("viewer-drag-n")).unwrap_or(0)));
         match g {
             Gesture::Pan { start_pan } => {
-                let d = resp.total_drag_delta().unwrap_or_default();
-                app.ui.viewer.pan = [start_pan[0] + d.x, start_pan[1] + d.y];
+                // No total drag on the release frame: keep the pan reached so far (it used to
+                // snap back to where the drag began).
+                if let Some(d) = resp.total_drag_delta() {
+                    app.ui.viewer.pan = [start_pan[0] + d.x, start_pan[1] + d.y];
+                }
             }
             Gesture::Move { layers, start, snap_src } => {
                 let mut d = [cpt[0] - start[0], cpt[1] - start[1]];

crates/ui-egui/tests/ui_viewer.rs

@@ -502,3 +502,35 @@ fn puppet_marquee_selects_pins_and_alt_drag_works_over_the_art() {
     let sel: Vec<u64> = h.state().session.state.selected_props.iter().map(|(_, u)| *u).collect();
     assert_eq!(sel, vec![bend]);
 }
+
+/// Drag with `button` from `from` to `to` in steps, then release.
+fn drag_with(h: &mut Harness<'_, EffectcraftApp>, button: egui::PointerButton, from: Pos2, to: Pos2) {
+    h.input_mut().events.push(Event::PointerMoved(from));
+    h.input_mut().events.push(Event::PointerButton { pos: from, button, pressed: true, modifiers: Default::default() });
+    h.step();
+    for i in 1..=8 {
+        h.input_mut().events.push(Event::PointerMoved(from + (to - from) * (i as f32 / 8.0)));
+        h.step();
+    }
+    h.input_mut().events.push(Event::PointerButton { pos: to, button, pressed: false, modifiers: Default::default() });
+    h.run_steps(3);
+}
+
+#[test]
+fn middle_and_hand_drags_pan_the_viewer_and_the_pan_stays_after_release() {
+    let mut h = harness();
+    let c = rect(&h, "viewer.comp").center();
+    let before = h.state().ui.viewer.pan;
+    // Middle-button drag with the Selection tool: pans, and stays panned after the release.
+    drag_with(&mut h, egui::PointerButton::Middle, c, c + vec2(120.0, 60.0));
+    let after = h.state().ui.viewer.pan;
+    assert!((after[0] - before[0] - 120.0).abs() < 1.0 && (after[1] - before[1] - 60.0).abs() < 1.0, "{before:?} → {after:?}");
+    assert_eq!(h.state().ui.tool, Tool::Selection, "the tool is unchanged");
+    // The Hand tool's primary drag keeps its pan too.
+    h.state_mut().ui.tool = Tool::Hand;
+    h.run_steps(2);
+    let c = rect(&h, "viewer.comp").center();
+    drag_with(&mut h, egui::PointerButton::Primary, c, c - vec2(50.0, 30.0));
+    let end = h.state().ui.viewer.pan;
+    assert!((end[0] - after[0] + 50.0).abs() < 1.0 && (end[1] - after[1] + 30.0).abs() < 1.0, "{after:?} → {end:?}");
+}