fix: undo closing a widget (#40)

Closing a widget was instant and destructive - one misclick on the X
lost its size and position. Close now shows a toast with an undo
button that re-adds the very same widget at its captured gs-x/y/w/h,
keeping its gs-id. Programmatic close() (fromClient=false) is skipped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HGRChzremwYCUctf2qMrQz

src/main/java/com/example/views/DashboardView.java

@@ -19,6 +19,7 @@ import com.vaadin.flow.component.button.ButtonVariant;
 import com.vaadin.flow.component.dialog.Dialog;
 import com.vaadin.flow.component.html.Paragraph;
 import com.vaadin.flow.component.html.Span;
+import com.vaadin.flow.component.notification.Notification;
 import com.vaadin.flow.component.orderedlayout.HorizontalLayout;
 import com.vaadin.flow.component.orderedlayout.VerticalLayout;
 import com.vaadin.flow.router.HasDynamicTitle;
@@ -40,7 +41,8 @@ import java.util.Optional;
  * layout is persisted per browser, plus controls to add widgets and reset the
  * layout at runtime. Widgets are removed by their own close button (see
  * {@link GridStackItem#setClosable(boolean)}), not from the toolbar, and are
- * added back from the picker over {@link WidgetRegistry}.
+ * added back from the picker over {@link WidgetRegistry} — or undone straight
+ * from the toast every close spawns (see {@link #offerUndo}).
  * <p>
  * All widget numbers come from {@link ChartDataService} and every chart widget
  * is built by the registry; this view only decides where a widget sits and
@@ -97,17 +99,18 @@ public class DashboardView extends VerticalLayout implements HasDynamicTitle {
                     refreshKpiTile(kpi.id());
                 }
             });
-            grid.add(item);
+            addWidgetToGrid(item);
         }
         context.addFilterChangeListener(this::updateKpiTiles);
 
-        grid.add(
+        List.of(
                 defaultWidget(WidgetRegistry.REVENUE_TREND, 0, 1, 6, 3),
                 defaultWidget(WidgetRegistry.REVENUE_MONTH, 6, 1, 6, 3),
                 defaultWidget(WidgetRegistry.REVENUE_REGION, 0, 4, 5, 3),
                 new GridStackItem("hint", 5, 4, 7, 3,
                         new Card(getTranslation("card.gridstackHint"),
-                                new Paragraph(getTranslation("gridstack.hint")))));
+                                new Paragraph(getTranslation("gridstack.hint")))))
+                .forEach(this::addWidgetToGrid);
 
         status.setText(getTranslation("gridstack.statusInitial"));
         status.addClassName("dialect-muted");
@@ -160,10 +163,57 @@ public class DashboardView extends VerticalLayout implements HasDynamicTitle {
         // widget must not hand its id to the next one, or the new widget would
         // inherit the closed one's saved position.
         extraWidgetCount++;
-        grid.add(widget(definition.type() + "-" + extraWidgetCount, definition,
+        addWidgetToGrid(widget(definition.type() + "-" + extraWidgetCount, definition,
                 0, 0, definition.width(), definition.height()));
     }
 
+    /** Adds an item to the grid and wires the undo toast onto its close — every
+     *  widget goes through here exactly once, so the close listener (and thus
+     *  the undo offer) is registered once per item, not once per re-add: an
+     *  undo hands the very same instance back to {@link #grid} directly. */
+    private void addWidgetToGrid(GridStackItem item) {
+        item.addCloseListener(this::offerUndo);
+        grid.add(item);
+    }
+
+    /**
+     * Closing a widget is one misclick away from losing its position, and a
+     * confirmation dialog on every close would be worse than the problem — so
+     * this shows a dismissible undo toast instead. Skipped for programmatic
+     * closes ({@link GridStackItem#close()}, e.g. from duplicate cleanup),
+     * which is exactly what {@link GridStackItem.CloseEvent#isFromClient()}
+     * tells apart.
+     * <p>
+     * Undo re-adds the very item that was closed — not a rebuilt copy — at the
+     * {@link GridStackItem.Position} the event captured before detaching, so it
+     * keeps its {@code gs-id} and its exact size/position rather than landing
+     * in the grid's next free slot.
+     */
+    private void offerUndo(GridStackItem.CloseEvent event) {
+        if (!event.isFromClient()) {
+            return;
+        }
+        GridStackItem item = event.getSource();
+        GridStackItem.Position position = event.getPosition();
+
+        Notification toast = new Notification();
+        toast.setDuration(8000);
+        toast.setPosition(Notification.Position.BOTTOM_START);
+
+        Button undo = new Button(getTranslation("gridstack.undo"), e -> {
+            item.setPosition(position.x(), position.y(), position.w(), position.h());
+            grid.add(item);
+            toast.close();
+        });
+        undo.addThemeVariants(ButtonVariant.LUMO_TERTIARY_INLINE);
+
+        HorizontalLayout content = new HorizontalLayout(
+                new Span(getTranslation("gridstack.closed")), undo);
+        content.setAlignItems(Alignment.CENTER);
+        toast.add(content);
+        toast.open();
+    }
+
     /** A widget of the initial set. Its id is the plain type — never handed out
      *  again by {@link #addWidget(WidgetDefinition)}, which always suffixes a
      *  counter — and it keeps the dashboard's own placement rather than the