docking: a tab dragged to an edge without a dock makes one

A tab that shares its group moves alone into the new dock with Action::MovePanel, as
window.panel.dock does (P6).

crates/ui/src/docking/dock_tests.rs

@@ -281,6 +281,16 @@ fn group_dropped_on_the_window_edge_without_a_dock_makes_a_new_dock() {
     }
 }
 
+#[test]
+fn tab_dragged_to_the_window_edge_without_a_dock_makes_a_dock_of_that_panel() {
+    let mut d = testing::DockDriver::new(docked(Side::Start, &[&[&[2, 3]]]), TestContent::default());
+    let tab = d.hit(&Hit::Tab(3)).unwrap_or_else(|| panic!("{:?}", d.hits()));
+    let out = d.drag(tab.center(), pos2(795.0, 300.0));
+    assert!(out.actions.contains(&Action::MovePanel { panel: 3, to: Target::NewDock { side: Side::End } }), "{:?}", out.actions);
+    assert_eq!(columns(&d.layout, Side::End), vec![vec![vec![3]]]);
+    assert_eq!(columns(&d.layout, Side::Start), vec![vec![vec![2]]]);
+}
+
 #[test]
 fn a_column_takes_groups_anywhere_in_it_and_as_tabs() {
     let windows = [(5, [400.0, 40.0, 150.0, 120.0]), (6, [400.0, 220.0, 150.0, 120.0]), (7, [400.0, 400.0, 150.0, 120.0])];

crates/ui/src/docking/render/dock.rs

@@ -461,8 +461,9 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
         self.checked(action).map(|action| (action, drop.line))
     }
 
-    /// The dock target a single tab drag would take: a new group or column of that panel. A new
-    /// dock only takes a panel that is alone in its group.
+    /// The dock target a single tab drag would take: a new group, column or dock of that panel.
+    /// A panel alone in its group takes the group's state to a new dock; one that shares its
+    /// group leaves the other tabs behind (P6).
     pub(super) fn dock_tab_proposal(&self, ctx: &Context, panel: &P, pointer: Pos2) -> Option<(Option<Action<P>>, Rect)> {
         if self.dock_covered(ctx, pointer, None) {
             return None;
@@ -476,7 +477,8 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
             {
                 Action::MoveGroup { panel: panel.clone(), to }
             }
-            Target::NewDock { .. } | Target::Float { .. } => return None,
+            to @ Target::NewDock { .. } => Action::MovePanel { panel: panel.clone(), to },
+            Target::Float { .. } => return None,
         };
         self.checked(action).map(|action| (action, drop.line))
     }