perf(grid): eliminate eager List<String> allocation on every pointer-move during drag selection

lib/features/workspace/grid/grid_pointer_selection.dart

@@ -4,25 +4,22 @@ part of '../result_grid_view.dart';
 
 /// Pointer, drag and tap selection for [VirtualResultGrid].
 extension _GridPointerSelection on _VirtualResultGridState {
-  void _notifySelectionAndFocus() {
+  void _notifySelectionAndFocus({bool deferValues = false}) {
     if (widget.onSelectionValuesChanged != null) {
       if (_selection == null) {
+        _selectionValuesDebounce?.cancel();
         widget.onSelectionValuesChanged!(const []);
+      } else if (deferValues) {
+        // During active drag gestures, defer allocating the multi-cell
+        // String list until the pointer stops moving or the drag ends (#1348).
+        _selectionValuesDebounce?.cancel();
+        _selectionValuesDebounce = Timer(const Duration(milliseconds: 40), () {
+          if (!mounted) return;
+          _dispatchSelectionValues();
+        });
       } else {
-        final rows = _sortedRows;
-        final values = <String>[];
-        for (var r = _selection!.startRow; r <= _selection!.endRow; r++) {
-          if (r >= 0 && r < rows.length) {
-            for (var c = _selection!.startColumn;
-                c <= _selection!.endColumn;
-                c++) {
-              if (c >= 0 && c < rows[r].length) {
-                values.add(_maskedValue(c, rows[r][c]));
-              }
-            }
-          }
-        }
-        widget.onSelectionValuesChanged!(values);
+        _selectionValuesDebounce?.cancel();
+        _dispatchSelectionValues();
       }
     }
 
@@ -39,6 +36,24 @@ extension _GridPointerSelection on _VirtualResultGridState {
     }
   }
 
+  void _dispatchSelectionValues() {
+    if (widget.onSelectionValuesChanged == null || _selection == null) return;
+    final rows = _sortedRows;
+    final values = <String>[];
+    for (var r = _selection!.startRow; r <= _selection!.endRow; r++) {
+      if (r >= 0 && r < rows.length) {
+        for (var c = _selection!.startColumn;
+            c <= _selection!.endColumn;
+            c++) {
+          if (c >= 0 && c < rows[r].length) {
+            values.add(_maskedValue(c, rows[r][c]));
+          }
+        }
+      }
+    }
+    widget.onSelectionValuesChanged!(values);
+  }
+
   ResultGridCellCoordinate? _cellAtOffset({
     required Offset localPosition,
     required double rowHeight,
@@ -127,7 +142,7 @@ extension _GridPointerSelection on _VirtualResultGridState {
           _focusNode.requestFocus();
           final modelRow = _toModelRowIndex(startCell.row);
           widget.onRowSelected?.call(modelRow);
-          _notifySelectionAndFocus();
+          _notifySelectionAndFocus(deferValues: true);
           setState(() {});
         }
         _startAutoScroll(rowHeight);
@@ -149,7 +164,7 @@ extension _GridPointerSelection on _VirtualResultGridState {
         });
         final modelRow = _toModelRowIndex(currentCell.row);
         widget.onRowSelected?.call(modelRow);
-        _notifySelectionAndFocus();
+        _notifySelectionAndFocus(deferValues: true);
       }
     }
   }
@@ -160,6 +175,7 @@ extension _GridPointerSelection on _VirtualResultGridState {
   }) {
     if (_isDragSelecting) {
       _hasJustDragSelected = true;
+      _notifySelectionAndFocus(deferValues: false);
       WidgetsBinding.instance.addPostFrameCallback((_) {
         _hasJustDragSelected = false;
       });
@@ -197,6 +213,9 @@ extension _GridPointerSelection on _VirtualResultGridState {
   }
 
   void _onGridPointerCancel(PointerCancelEvent event) {
+    if (_isDragSelecting) {
+      _notifySelectionAndFocus(deferValues: false);
+    }
     _isPointerDown = false;
     _isDragSelecting = false;
     _stopAutoScroll();
@@ -288,7 +307,7 @@ extension _GridPointerSelection on _VirtualResultGridState {
       });
       final modelRow = _toModelRowIndex(currentCell.row);
       widget.onRowSelected?.call(modelRow);
-      _notifySelectionAndFocus();
+      _notifySelectionAndFocus(deferValues: true);
     }
   }
 

lib/features/workspace/result_grid_view.dart

@@ -150,6 +150,7 @@ class _VirtualResultGridState extends material.State<VirtualResultGrid> {
   DateTime? _lastTapUpTime;
   ResultGridCellCoordinate? _lastTapCell;
   Timer? _autoScrollTimer;
+  Timer? _selectionValuesDebounce;
   List<double> _currentDisplayOffsets = const [0];
   double _currentAvailableWidth = 0.0;
 
@@ -260,6 +261,7 @@ class _VirtualResultGridState extends material.State<VirtualResultGrid> {
 
   @override
   void dispose() {
+    _selectionValuesDebounce?.cancel();
     PiiMaskingController.instance.removeListener(_onPiiMaskingChanged);
     _stopAutoScroll();
     _rowWidgets.clear();

test/features/workspace/result_grid_drag_selection_test.dart

@@ -54,6 +54,59 @@ void main() {
       expect(selectedValues, ['1', 'Alice', '2', 'Bob']);
     });
 
+    testWidgets('does not eagerly allocate and dispatch selection values on every intermediate drag move', (tester) async {
+      var notifications = 0;
+      List<String>? lastValues;
+
+      await tester.pumpWidget(
+        _testShell(
+          child: material.SizedBox(
+            width: 800,
+            height: 400,
+            child: VirtualResultGrid(
+              columns: const ['id', 'name', 'role'],
+              rows: const [
+                ['1', 'Alice', 'Admin'],
+                ['2', 'Bob', 'User'],
+                ['3', 'Charlie', 'Manager'],
+              ],
+              onSelectionValuesChanged: (vals) {
+                notifications++;
+                lastValues = vals;
+              },
+            ),
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      final startPos = tester.getCenter(find.text('1'));
+      final cellAlice = tester.getCenter(find.text('Alice'));
+      final cellBob = tester.getCenter(find.text('Bob'));
+
+      final gesture = await tester.startGesture(startPos, kind: PointerDeviceKind.mouse);
+      await tester.pump();
+      final initialNotifications = notifications;
+
+      // Move across Alice without releasing pointer or waiting for debounce
+      await gesture.moveTo(cellAlice);
+      await tester.pump();
+
+      // Move across Bob without releasing pointer
+      await gesture.moveTo(cellBob);
+      await tester.pump();
+
+      // Intermediate drag-move steps must not trigger synchronous value list extractions (#1348)
+      expect(notifications, equals(initialNotifications));
+
+      // Releasing pointer flushes final selection values immediately
+      await gesture.up();
+      await tester.pump();
+
+      expect(notifications, greaterThan(initialNotifications));
+      expect(lastValues, ['1', 'Alice', '2', 'Bob']);
+    });
+
     testWidgets('supports reverse drag selection (bottom-right to top-left)', (tester) async {
       List<String>? selectedValues;