Merge PR #769 from switch87: Preferences: Interface size applies when the slider is let go

crates/ui-egui/src/dialogs.rs

@@ -2142,9 +2142,21 @@ fn body(app: &mut SlideApp, ui: &mut Ui, d: &mut Dialog) -> bool {
             if ui.checkbox(&mut dark, "Dark appearance").changed() {
                 let _ = app.run("view.dark", json!({"on": dark}));
             }
-            let mut scale = app.ui.ui_scale;
-            if ui.add(egui::Slider::new(&mut scale, 0.75..=1.75).text("Interface size")).changed() {
-                app.ui.ui_scale = scale;
+            // While the pointer holds the slider its value only waits in the dialog: applying it
+            // would rescale the slider under the pointer and run it to an end (#767). It applies
+            // on release, and at once from the keyboard.
+            let pending = d.fields.get("uiScale").and_then(|v| v.parse::<f32>().ok());
+            let mut scale = pending.unwrap_or(app.ui.ui_scale);
+            let r = ui.add(egui::Slider::new(&mut scale, 0.75..=1.75).text("Interface size"));
+            if r.is_pointer_button_down_on() {
+                if r.changed() || pending.is_some() {
+                    d.fields.insert("uiScale".into(), scale.to_string());
+                }
+            } else {
+                if r.changed() || pending.is_some() {
+                    app.ui.ui_scale = scale;
+                }
+                d.fields.remove("uiScale");
             }
             ui.label(egui::RichText::new("Editing").font(theme::bold(13.0)));
             ui.checkbox(&mut app.session.prefs.smart_guides, "Show smart guides");

crates/ui-egui/src/lib.rs

@@ -3322,4 +3322,38 @@ mod tests {
         flick(&mut harness, egui::vec2(-24.0, 0.0));
         assert_eq!(harness.state().show.as_ref().map(|s| s.state.slide), Some(0));
     }
+
+    /// Preferences › Interface size (#767): dragging the slider leaves the interface size alone
+    /// until the pointer lets go, so the slider doesn't grow under the pointer mid-drag.
+    #[test]
+    fn interface_size_applies_when_the_slider_is_let_go() {
+        let mut harness = egui_kittest::Harness::builder().with_size(egui::vec2(1200.0, 800.0)).build_ui_state(
+            |ui, app: &mut SlideApp| {
+                app.logic(ui.ctx());
+                app.ui(ui);
+            },
+            SlideApp::new(Session::with_new(), Services::default()),
+        );
+        harness.run_steps(2);
+        harness.state_mut().run("app.preferences", json!({})).unwrap();
+        harness.run_steps(4);
+        let before = harness.state().ui.ui_scale;
+        let rect =
+            harness.query_all_by_role_and_label(egui::accesskit::Role::Slider, "Interface size").next().expect("the Interface size slider").rect();
+        let (from, to) = (rect.left_center() + egui::vec2(4.0, 0.0), rect.right_center() - egui::vec2(4.0, 0.0));
+        harness.hover_at(from);
+        harness.run_steps(1);
+        harness.drag_at(from);
+        harness.run_steps(1);
+        for k in 1..=8 {
+            harness.hover_at(from.lerp(to, k as f32 / 8.0));
+            harness.run_steps(1);
+        }
+        assert_eq!(harness.state().ui.ui_scale, before, "nothing rescales while the slider is held");
+        harness.drop_at(to);
+        harness.run_steps(2);
+        let after = harness.state().ui.ui_scale;
+        assert!(after > before + 0.3, "letting go applies the size dragged to: {before} -> {after}");
+        assert!(harness.state().dialog.is_some(), "Preferences stays open");
+    }
 }