Fix a bug where ScrollableGrid/ScrollableList would flicker when (#4794)

navigating away.

Details are in the bug, but when we navigate away, the overscroll
indicator around the Scrollable would change the widget hierarchy. This
would trigger the Scrollable to rebuild, which would cause its
scrollOffset to be clamped to 0 for a frame.

BUG=https://github.com/flutter/flutter/issues/4597

packages/flutter/lib/src/material/overscroll_indicator.dart

@@ -226,10 +226,10 @@ class _OverscrollIndicatorState extends State<OverscrollIndicator> {
       child: new AnimatedBuilder(
         animation: _extentAnimation,
         builder: (BuildContext context, Widget child) {
-          if (_scrollDirection == null) // Haven't seen a scroll yet.
-            return child;
+          // We keep the same widget hierarchy here, even when we're not
+          // painting anything, to avoid rebuilding the children.
           return new CustomPaint(
-            foregroundPainter: new _Painter(
+            foregroundPainter: _scrollDirection == null ? null : new _Painter(
               scrollDirection: _scrollDirection,
               extent: _extentAnimation.value,
               isLeading: _scrollOffset < _minScrollOffset,

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

@@ -1610,8 +1610,8 @@ class RenderCustomPaint extends RenderProxyBox {
       markNeedsPaint();
     }
     if (attached) {
-      oldPainter._repaint?.removeListener(markNeedsPaint);
-      newPainter._repaint?.addListener(markNeedsPaint);
+      oldPainter?._repaint?.removeListener(markNeedsPaint);
+      newPainter?._repaint?.addListener(markNeedsPaint);
     }
   }
 

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

@@ -50,6 +50,10 @@ class ClampOverscrolls extends InheritedWidget {
   ///
   /// This utility function is typically used by [Scrollable.builder] callbacks.
   static Widget buildViewport(BuildContext context, ScrollableState state, ViewportBuilder builder) {
+    // TODO(ianh): minScrollOffset and maxScrollOffset are typically determined
+    // by the container and content size. But we don't know those until we
+    // layout the viewport, which happens after build phase. We need to rethink
+    // this.
     final bool clampOverscrolls = ClampOverscrolls.of(context);
     final double clampedScrollOffset = clampOverscrolls
       ? state.scrollOffset.clamp(state.scrollBehavior.minScrollOffset, state.scrollBehavior.maxScrollOffset)