Fix a bug with how ScrollableGrid deals with padding. (#5963)

It wasn't taking padding into account when deciding which children were
visible. I modeled the solution off of the way ScrollableList handles
this.

Fixes https://github.com/flutter/flutter/issues/5522

packages/flutter/lib/src/rendering/grid.dart

@@ -205,6 +205,9 @@ abstract class GridDelegate {
     return getGridSpecification(constraints, childCount).gridSize;
   }
 
+  /// Insets for the entire grid.
+  EdgeInsets get padding => EdgeInsets.zero;
+
   // TODO(ianh): It's a bit dubious to be using the getSize function from the delegate to
   // figure out the intrinsic dimensions. We really should either not support intrinsics,
   // or we should expose intrinsic delegate callbacks and throw if they're not implemented.
@@ -301,6 +304,7 @@ abstract class GridDelegateWithInOrderChildPlacement extends GridDelegate {
   final double rowSpacing;
 
   /// Insets for the entire grid.
+  @override
   final EdgeInsets padding;
 
   @override
@@ -637,8 +641,18 @@ class RenderGrid extends RenderVirtualViewport<GridParentData> {
     if (callback != null)
       invokeLayoutCallback(callback);
 
-    final double gridTopPadding = _specification.padding.top;
-    final double gridLeftPadding = _specification.padding.left;
+    double gridTopPadding = 0.0;
+    double gridLeftPadding = 0.0;
+
+    switch (mainAxis) {
+      case Axis.vertical:
+        gridLeftPadding = _specification.padding.left;
+        break;
+      case Axis.horizontal:
+        gridTopPadding = _specification.padding.top;
+        break;
+    }
+
     int childIndex = virtualChildBase;
     RenderBox child = firstChild;
     while (child != null) {

packages/flutter/lib/src/widgets/scrollable_grid.dart

@@ -83,7 +83,7 @@ class ScrollableGrid extends StatelessWidget {
 
   Widget _buildViewport(BuildContext context, ScrollableState state, double scrollOffset) {
     return new GridViewport(
-      startOffset: scrollOffset,
+      scrollOffset: scrollOffset,
       delegate: delegate,
       onExtentsChanged: state.handleExtentsChanged,
       children: children
@@ -126,7 +126,7 @@ class GridViewport extends VirtualViewportFromIterable {
   ///
   /// The [delegate] argument must not be null.
   GridViewport({
-    this.startOffset,
+    this.scrollOffset,
     this.delegate,
     this.onExtentsChanged,
     this.children
@@ -134,8 +134,15 @@ class GridViewport extends VirtualViewportFromIterable {
     assert(delegate != null);
   }
 
+  /// The [startOffset] without taking the [delegate]'s padding into account.
+  final double scrollOffset;
+
   @override
-  final double startOffset;
+  double get startOffset {
+    if (delegate == null)
+      return scrollOffset;
+    return scrollOffset - delegate.padding.top;
+  }
 
   /// The delegate that controls the layout of the children.
   final GridDelegate delegate;

packages/flutter/test/widget/scrollable_grid_test.dart

@@ -0,0 +1,67 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter/rendering.dart';
+
+void main() {
+  // Tests https://github.com/flutter/flutter/issues/5522
+  testWidgets('ScrollableGrid displays correct children with nonzero padding', (WidgetTester tester) async {
+    GlobalKey<ScrollableState> scrollableKey = new GlobalKey<ScrollableState>();
+    final EdgeInsets padding = new EdgeInsets.fromLTRB(0.0, 100.0, 0.0, 0.0);
+
+    Widget testWidget = new Align(
+      child: new SizedBox(
+        height: 800.0,
+        width: 300.0,  // forces the grid children to be 300..300
+        child: new ScrollableGrid(
+          scrollableKey: scrollableKey,
+          delegate: new FixedColumnCountGridDelegate(
+            columnCount: 1,
+            padding: padding
+          ),
+          children: new List<Widget>.generate(10, (int index) {
+            return new Text('$index', key: new ValueKey<int>(index));
+          })
+        )
+      )
+    );
+
+    await tester.pumpWidget(testWidget);
+
+    // screen is 600px high, and has the following items:
+    //   100..400 = 0
+    //   400..700 = 1
+    await tester.pump();
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('1'), findsOneWidget);
+    expect(find.text('2'), findsNothing);
+    expect(find.text('3'), findsNothing);
+
+    await tester.scroll(find.text('1'), const Offset(0.0, -500.0));
+    await tester.pump();
+    //  -100..300 = 1
+    //   300..600 = 2
+    //   600..600 = 3
+    expect(find.text('0'), findsNothing);
+    expect(find.text('1'), findsOneWidget);
+    expect(find.text('2'), findsOneWidget);
+    expect(find.text('3'), findsOneWidget);
+    expect(find.text('4'), findsNothing);
+    expect(find.text('5'), findsNothing);
+
+    await tester.scroll(find.text('1'), const Offset(0.0, 150.0));
+    await tester.pump();
+    // Child '0' is now back onscreen, but by less than `padding.top`.
+    //  -250..050 = 0
+    //   050..450 = 1
+    //   450..750 = 2
+    expect(find.text('0'), findsOneWidget);
+    expect(find.text('1'), findsOneWidget);
+    expect(find.text('2'), findsOneWidget);
+    expect(find.text('3'), findsNothing);
+    expect(find.text('4'), findsNothing);
+  });
+}