Do not apply scroll offset corrections with zero values during sliver list layout (#10574)

Fixes https://github.com/flutter/flutter/issues/10547

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

@@ -485,13 +485,13 @@ class SliverGeometry {
     double hitTestExtent,
     bool visible,
     this.hasVisualOverflow: false,
-    this.scrollOffsetCorrection: 0.0
+    this.scrollOffsetCorrection,
   }) : assert(scrollExtent != null),
        assert(paintExtent != null),
        assert(paintOrigin != null),
        assert(maxPaintExtent != null),
        assert(hasVisualOverflow != null),
-       assert(scrollOffsetCorrection != null),
+       assert(scrollOffsetCorrection != 0.0),
        layoutExtent = layoutExtent ?? paintExtent,
        hitTestExtent = hitTestExtent ?? paintExtent,
        visible = visible ?? paintExtent > 0.0;
@@ -613,7 +613,7 @@ class SliverGeometry {
       verify(hitTestExtent >= 0.0, 'The "hitTestExtent" is negative.');
       verify(visible != null, 'The "visible" property is null.');
       verify(hasVisualOverflow != null, 'The "hasVisualOverflow" is null.');
-      verify(scrollOffsetCorrection != null, 'The "scrollOffsetCorrection" is null.');
+      verify(scrollOffsetCorrection != 0.0, 'The "scrollOffsetCorrection" is zero.');
       return true;
     });
     return true;
@@ -648,7 +648,8 @@ class SliverGeometry {
         buffer.write('hitTestExtent: ${hitTestExtent.toStringAsFixed(1)}, ');
       if (hasVisualOverflow)
         buffer.write('hasVisualOverflow: true, ');
-      buffer.write('scrollOffsetCorrection: ${scrollOffsetCorrection.toStringAsFixed(1)}');
+      if (scrollOffsetCorrection != null)
+        buffer.write('scrollOffsetCorrection: ${scrollOffsetCorrection.toStringAsFixed(1)}');
     buffer.write(')');
     return buffer.toString();
   }

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

@@ -99,15 +99,23 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
       earliestUsefulChild = insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
 
       if (earliestUsefulChild == null) {
-        // We ran out of children before reaching the scroll offset.
-        // We must inform our parent that this sliver cannot fulfill
-        // its contract and that we need a scroll offset correction.
-        geometry = new SliverGeometry(
-          scrollOffsetCorrection: -scrollOffset,
-        );
         final SliverMultiBoxAdaptorParentData childParentData = firstChild.parentData;
         childParentData.layoutOffset = 0.0;
-        return;
+
+        if (scrollOffset == 0.0) {
+          earliestUsefulChild = firstChild;
+          leadingChildWithLayout = earliestUsefulChild;
+          trailingChildWithLayout ??= earliestUsefulChild;
+          break;
+        } else {
+          // We ran out of children before reaching the scroll offset.
+          // We must inform our parent that this sliver cannot fulfill
+          // its contract and that we need a scroll offset correction.
+          geometry = new SliverGeometry(
+            scrollOffsetCorrection: -scrollOffset,
+          );
+          return;
+        }
       }
 
       final double firstChildScrollOffset = earliestScrollOffset - paintExtentOf(firstChild);

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

@@ -260,7 +260,7 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
       assert(childLayoutGeometry.debugAssertIsValid);
 
       // If there is a correction to apply, we'll have to start over.
-      if (childLayoutGeometry.scrollOffsetCorrection != 0.0)
+      if (childLayoutGeometry.scrollOffsetCorrection != null)
         return childLayoutGeometry.scrollOffsetCorrection;
 
       // We use the child's paint origin in our coordinate system as the

packages/flutter/test/rendering/slivers_block_test.dart

@@ -26,7 +26,6 @@ class TestRenderSliverBoxChildManager extends RenderSliverBoxChildManager {
 
   @override
   void createChild(int index, { @required RenderBox after }) {
-    assert(index >= 0);
     if (index < 0 || index >= children.length)
       return null;
     try {
@@ -213,4 +212,36 @@ void main() {
     expect(e.attached, false);
   });
 
+  test('SliverList - no zero scroll offset correction', () {
+    RenderSliverList inner;
+    RenderBox a;
+    final TestRenderSliverBoxChildManager childManager = new TestRenderSliverBoxChildManager(
+      children: <RenderBox>[
+        a = new RenderSizedBox(const Size(100.0, 400.0)),
+        new RenderSizedBox(const Size(100.0, 400.0)),
+        new RenderSizedBox(const Size(100.0, 400.0)),
+        new RenderSizedBox(const Size(100.0, 400.0)),
+        new RenderSizedBox(const Size(100.0, 400.0)),
+      ],
+    );
+    final RenderViewport root = new RenderViewport(
+      axisDirection: AxisDirection.down,
+      offset: new ViewportOffset.zero(),
+      children: <RenderSliver>[
+        inner = childManager.createRenderObject(),
+      ],
+    );
+    layout(root);
+
+    final SliverMultiBoxAdaptorParentData parentData = a.parentData;
+    parentData.layoutOffset = 0.001;
+
+    root.offset = new ViewportOffset.fixed(900.0);
+    pumpFrame();
+
+    root.offset = new ViewportOffset.fixed(0.0);
+    pumpFrame();
+
+    expect(inner.geometry.scrollOffsetCorrection, isNull);
+  });
 }