Exclude SemanticNodes that are fully covered by an overlapping sliver (#11826)

* Exclude SemanticNodes that are fully covered by an overlapping sliver

* simplification

* formatting

* nits

* add test for center widget

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

@@ -285,6 +285,36 @@ abstract class RenderSliverMultiBoxAdaptor extends RenderSliver
       visitor(child);
   }
 
+  @override
+  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
+    switch (constraints.normalizedGrowthDirection) {
+      case GrowthDirection.forward:
+        super.visitChildrenForSemantics((RenderObject child) {
+          // The sliver is overlapped at the leading edge.
+          final Offset bottomLeftInViewport = MatrixUtils.transformPoint(
+              child.getTransformTo(parent), child.semanticBounds.bottomLeft
+          );
+          final double endOverlap = constraints.overlap;
+          if ((constraints.axis == Axis.vertical && bottomLeftInViewport.dy > endOverlap) ||
+              (constraints.axis == Axis.horizontal && bottomLeftInViewport.dx > endOverlap))
+            visitor(child);
+        });
+        break;
+      case GrowthDirection.reverse:
+        super.visitChildrenForSemantics((RenderObject child) {
+          // The sliver is overlapped at the trailing edge.
+          final Offset topRightInViewport = MatrixUtils.transformPoint(
+              child.getTransformTo(parent), child.semanticBounds.topRight
+          );
+          final double startOverlap = constraints.remainingPaintExtent - constraints.overlap;
+          if ((constraints.axis == Axis.vertical && topRightInViewport.dy < startOverlap) ||
+              (constraints.axis == Axis.horizontal && topRightInViewport.dx < startOverlap))
+            visitor(child);
+        });
+        break;
+    }
+  }
+
   /// Called during layout to create and add the child with the given index and
   /// scroll offset.
   ///

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

@@ -97,7 +97,8 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
   @override
   void visitChildrenForSemantics(RenderObjectVisitor visitor) {
     for (RenderSliver sliver in childrenInPaintOrder) {
-      if (sliver.geometry.paintExtent != 0)
+      if (sliver.geometry.paintExtent != 0 &&
+          sliver.constraints.overlap < sliver.geometry.paintOrigin + sliver.geometry.paintExtent)
         visitor(sliver);
     }
   }