Keep disabled scroll animations in Preferences (#5886)

crates/ui-egui/src/dialogs.rs

@@ -20,6 +20,9 @@ pub(super) mod protect;
 pub(super) mod standards_ui;
 pub(super) mod word_count_ui;
 
+#[cfg(test)]
+mod tests;
+
 pub(crate) use prompts::{PromptButton, save_prompt_key};
 use prompts::{full_screen_prompt, link_prompt, password, remote_link_prompt, save_prompt, unlock};
 
@@ -1371,20 +1374,26 @@ fn dialog_preferences(ui: &mut egui::Ui, app: &mut PdfCraftApp, req: &mut Pendin
     // (which only applies when the settings overflow it) opens the dialog at full size.
     // A scrollbar of its own: the floating one lies over the right edge of the theme
     // cards and other full-width rows (#3557).
+    preferences_scroll(ui, max, |ui| preferences::preferences_body(ui, app, &t));
+    req.close = preferences::preferences_buttons(ui);
+    false
+}
+
+fn preferences_scroll(ui: &mut egui::Ui, max: f32, add_contents: impl FnOnce(&mut egui::Ui)) {
     ui.scope(|ui| {
         ui.spacing_mut().scroll.floating = false;
         ui.spacing_mut().scroll.bar_inner_margin = 14.0;
         ui.spacing_mut().scroll.bar_outer_margin = 2.0;
-        ui.style_mut().scroll_animation = egui::style::ScrollAnimation::duration(0.25);
+        if ui.style().scroll_animation != egui::style::ScrollAnimation::none() {
+            ui.style_mut().scroll_animation = egui::style::ScrollAnimation::duration(0.25);
+        }
         egui::ScrollArea::vertical()
             .id_salt("preferences-dialog-scroll")
             .max_height(max)
             .min_scrolled_height(max)
             .animated(true)
-            .show(ui, |ui| preferences::preferences_body(ui, app, &t));
+            .show(ui, add_contents);
     });
-    req.close = preferences::preferences_buttons(ui);
-    false
 }
 
 /// `Dialog::NumberPages` content. Returns `false` when it has its own buttons and no footer is wanted.

crates/ui-egui/src/dialogs/tests.rs

@@ -0,0 +1,35 @@
+use std::cell::Cell;
+use std::rc::Rc;
+
+use egui_kittest::Harness;
+use egui_kittest::kittest::Queryable;
+
+#[test]
+fn preferences_scroll_preserves_disabled_animation() {
+    for animated in [false, true] {
+        let scroll = Rc::new(Cell::new(false));
+        let requested = scroll.clone();
+        let mut h = Harness::builder().with_size(egui::vec2(600.0, 400.0)).with_step_dt(1.0 / 60.0).build_ui(move |ui| {
+            super::preferences_scroll(ui, 200.0, |ui| {
+                ui.add_space(1000.0);
+                let target = ui.label("Scroll target");
+                if requested.replace(false) {
+                    ui.scroll_to_rect(target.rect, Some(egui::Align::BOTTOM));
+                }
+                ui.add_space(300.0);
+            });
+        });
+        h.ctx.all_styles_mut(|style| {
+            style.scroll_animation = if animated { egui::style::ScrollAnimation::default() } else { egui::style::ScrollAnimation::none() };
+        });
+        h.run_steps(4);
+        let before = h.get_by_label("Scroll target").rect().top();
+        scroll.set(true);
+        h.run_steps(4);
+        let early = h.get_by_label("Scroll target").rect().top();
+        h.run_steps(20);
+        let settled = h.get_by_label("Scroll target").rect().top();
+        assert!((before - settled).abs() > 50.0, "the target must scroll: {before} -> {settled}");
+        assert_eq!((early - settled).abs() > 1.0, animated, "animated: {animated}, early: {early}, settled: {settled}");
+    }
+}