Test #46: panels dock by a drag from the panel menu button; gutters resize

Covers what issue #46 reports (v0.1.1 on Linux): a panel dragged by its hamburger onto another
group's edge docks there as its own group, and dragging the gap between panels resizes them.
Both work on main (tab dragging was reworked in M3.15 for v0.2.0); this keeps them working.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/tests/ui_tabs.rs

@@ -166,3 +166,36 @@ fn nested_comp_markers_show_on_the_precomp_bar() {
     // (29.97 fps: the marker's frame is at 1.001 s)
     assert!((h.state().session.time().seconds() - 1.0).abs() < 0.034, "{}", h.state().session.time().seconds());
 }
+
+/// Issue #46: a panel dragged by its panel-menu (hamburger) button onto the left edge of another
+/// group docks there as its own group; dragging the gap between panels resizes them.
+#[test]
+fn panels_dock_beside_others_and_gutters_resize_them() {
+    let (mut h, _, _) = harness();
+    let dock = |h: &Harness<'_, EffectcraftApp>| h.state().ui.dock.clone();
+    // Show Effect Controls in its group so its menu button is on screen, then drag by it.
+    let tab = rect(&h, "panel.tab.EffectControls");
+    click_n(&mut h, tab.center(), 1);
+    let grip = rect(&h, "panel.menu.EffectControls").center();
+    let tl = rect(&h, "panel.Timeline");
+    drag(&mut h, grip, pos2(tl.min.x + tl.width() * 0.08, tl.center().y));
+    assert_eq!(group_of(&dock(&h), PanelKind::EffectControls), Some(vec![PanelKind::EffectControls]));
+    assert!(!group_of(&dock(&h), PanelKind::Timeline).unwrap().contains(&PanelKind::EffectControls));
+    let (fx, tl) = (rect(&h, "panel.EffectControls"), rect(&h, "panel.Timeline"));
+    assert!(fx.max.x <= tl.min.x + 1.0 && (fx.center().y - tl.center().y).abs() < tl.height(), "{fx:?} {tl:?}");
+    // The gutter on the Project panel's right edge: dragging it 60 px widens the panel by 60.
+    let project = rect(&h, "panel.Project");
+    let gutter = h
+        .state()
+        .auto
+        .previous
+        .iter()
+        .filter(|e| e.id.starts_with("dock.gutter.") && e.rect[3] > e.rect[2])
+        .map(|e| egui::Rect::from_min_size(pos2(e.rect[0], e.rect[1]), egui::vec2(e.rect[2], e.rect[3])))
+        .filter(|g| g.y_range().contains(project.center().y))
+        .min_by(|a, b| (a.center().x - project.max.x).abs().total_cmp(&(b.center().x - project.max.x).abs()))
+        .expect("a gutter beside the Project panel");
+    drag(&mut h, gutter.center(), gutter.center() + egui::vec2(60.0, 0.0));
+    let wider = rect(&h, "panel.Project");
+    assert!((wider.width() - project.width() - 60.0).abs() < 2.0, "{} → {}", project.width(), wider.width());
+}