Make getOffsetToReveal work with nested Viewports (#26663)

Fixes #20893, the most-upvoted bug for framework.

A detailed analysis explaining the cause of the bug has been posted at #20893 (comment).

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

@@ -57,9 +57,9 @@ abstract class RenderAbstractViewport extends RenderObject {
   /// edge of the viewport as possible. If `alignment` is 0.5, the child must be
   /// positioned as close to the center of the viewport as possible.
   ///
-  /// The target might not be a direct child of this viewport but it must be a
-  /// descendant of the viewport and there must not be any other
-  /// [RenderAbstractViewport] objects between the target and this object.
+  /// The `target` might not be a direct child of this viewport but it must be a
+  /// descendant of the viewport. Other viewports in between this viewport and
+  /// the `target` will not be adjusted.
   ///
   /// This method assumes that the content of the viewport moves linearly, i.e.
   /// when the offset of the viewport is changed by x then `target` also moves
@@ -585,26 +585,39 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
 
   @override
   RevealedOffset getOffsetToReveal(RenderObject target, double alignment, {Rect rect}) {
-    double leadingScrollOffset;
+    double leadingScrollOffset = 0.0;
     double targetMainAxisExtent;
-    RenderObject descendant;
     rect ??= target.paintBounds;
 
-    if (target is RenderBox) {
-      final RenderBox targetBox = target;
-
-      // The pivot will be the topmost child before we hit a RenderSliver.
-      RenderBox pivot = targetBox;
-      while (pivot.parent is RenderBox)
-        pivot = pivot.parent;
+    // Starting at `target` and walking towards the root:
+    //  - `child` will be the last object before we reach this viewport, and
+    //  - `pivot` will be the last RenderBox before we reach this viewport.
+    RenderObject child = target;
+    RenderBox pivot;
+    bool onlySlivers = target is RenderSliver; // ... between viewport and `target` (`target` included).
+    while (child.parent != this) {
+      assert(child.parent != null, '$target must be a descendant of $this');
+      if (child is RenderBox) {
+        pivot = child;
+      }
+      if (child.parent is RenderSliver) {
+        final RenderSliver parent = child.parent;
+        leadingScrollOffset += parent.childScrollOffset(child);
+      } else {
+        onlySlivers = false;
+        leadingScrollOffset = 0.0;
+      }
+      child = child.parent;
+    }
 
+    if (pivot != null) {
       assert(pivot.parent != null);
       assert(pivot.parent != this);
       assert(pivot != this);
       assert(pivot.parent is RenderSliver);  // TODO(abarth): Support other kinds of render objects besides slivers.
       final RenderSliver pivotParent = pivot.parent;
 
-      final Matrix4 transform = targetBox.getTransformTo(pivot);
+      final Matrix4 transform = target.getTransformTo(pivot);
       final Rect bounds = MatrixUtils.transformRect(transform, rect);
 
       final GrowthDirection growthDirection = pivotParent.constraints.growthDirection;
@@ -619,15 +632,15 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
               offset = bounds.top;
               break;
           }
-          leadingScrollOffset = pivot.size.height - offset;
+          leadingScrollOffset += pivot.size.height - offset;
           targetMainAxisExtent = bounds.height;
           break;
         case AxisDirection.right:
-          leadingScrollOffset = bounds.left;
+          leadingScrollOffset += bounds.left;
           targetMainAxisExtent = bounds.width;
           break;
         case AxisDirection.down:
-          leadingScrollOffset = bounds.top;
+          leadingScrollOffset += bounds.top;
           targetMainAxisExtent = bounds.height;
           break;
         case AxisDirection.left:
@@ -640,28 +653,17 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
               offset = bounds.left;
               break;
           }
-          leadingScrollOffset = pivot.size.width - offset;
+          leadingScrollOffset += pivot.size.width - offset;
           targetMainAxisExtent = bounds.width;
           break;
       }
-      descendant = pivot;
-    } else if (target is RenderSliver) {
+    } else if (onlySlivers) {
       final RenderSliver targetSliver = target;
-      leadingScrollOffset = 0.0;
       targetMainAxisExtent = targetSliver.geometry.scrollExtent;
-      descendant = targetSliver;
     } else {
       return RevealedOffset(offset: offset.pixels, rect: rect);
     }
 
-    // The child will be the topmost object before we get to the viewport.
-    RenderObject child = descendant;
-    while (child.parent is RenderSliver) {
-      final RenderSliver parent = child.parent;
-      leadingScrollOffset += parent.childScrollOffset(child);
-      child = parent;
-    }
-
     assert(child.parent == this);
     assert(child is RenderSliver);
     final RenderSliver sliver = child;

packages/flutter/test/rendering/viewport_test.dart

@@ -642,6 +642,118 @@ void main() {
     });
   });
 
+  testWidgets('Nested Viewports showOnScreen with allowImplicitScrolling=false for inner viewport', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/20893.
+
+    List<Widget> slivers;
+    final ScrollController controllerX =  ScrollController(initialScrollOffset: 0.0);
+    final ScrollController controllerY  = ScrollController(initialScrollOffset: 0.0);
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: Container(
+            height: 200.0,
+            width: 200.0,
+            child: ListView(
+              controller: controllerY,
+              children: <Widget>[
+                Container(
+                  height: 150.0,
+                ),
+                Container(
+                  height: 100.0,
+                  child: ListView(
+                    physics: const PageScrollPhysics(), // Turns off `allowImplicitScrolling`
+                    scrollDirection: Axis.horizontal,
+                    controller: controllerX,
+                    children: slivers = <Widget>[
+                      Container(
+                        width: 150.0,
+                      ),
+                      Container(
+                        width: 150.0,
+                      ),
+                    ],
+                  ),
+                ),
+                Container(
+                  height: 150.0,
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    tester.renderObject(find.byWidget(slivers[1])).showOnScreen();
+    await tester.pumpAndSettle();
+    expect(controllerX.offset, 0.0);
+    expect(controllerY.offset, 50.0);
+  });
+
+  testWidgets('Nested Viewports showOnScreen on Sliver with allowImplicitScrolling=false for inner viewport', (WidgetTester tester) async {
+    Widget sliver;
+    final ScrollController controllerX =  ScrollController(initialScrollOffset: 0.0);
+    final ScrollController controllerY  = ScrollController(initialScrollOffset: 0.0);
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: Container(
+            height: 200.0,
+            width: 200.0,
+            child: ListView(
+              controller: controllerY,
+              children: <Widget>[
+                Container(
+                  height: 150.0,
+                ),
+                Container(
+                  height: 100.0,
+                  child: CustomScrollView(
+                    physics: const PageScrollPhysics(), // Turns off `allowImplicitScrolling`
+                    scrollDirection: Axis.horizontal,
+                    controller: controllerX,
+                    slivers: <Widget>[
+                      SliverPadding(
+                        padding: const EdgeInsets.all(25.0),
+                        sliver: SliverToBoxAdapter(
+                          child: Container(
+                            width: 100.0,
+                          ),
+                        ),
+                      ),
+                      SliverPadding(
+                        padding: const EdgeInsets.all(25.0),
+                        sliver: sliver = SliverToBoxAdapter(
+                          child: Container(
+                            width: 100.0,
+                          ),
+                        ),
+                      ),
+                    ],
+                  ),
+                ),
+                Container(
+                  height: 150.0,
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    tester.renderObject(find.byWidget(sliver)).showOnScreen();
+    await tester.pumpAndSettle();
+    expect(controllerX.offset, 0.0);
+    expect(controllerY.offset, 25.0);
+  });
+
   testWidgets('Viewport showOnScreen with objects larger than viewport', (WidgetTester tester) async {
     List<Widget> children;
     ScrollController controller;