Few overscroll glow fixes. (#4610)

Fixes #4169 and also now overscroll indicator is dismissed the moment
user scrolls in the opposite direction (#4603) but the bounce overscroll
that happens behind the scene and is clamped in the indicator is still
problematic and needs to be fixed. However these fixes are orthogonal to that.

Also closes #4127 as I verified the timeout feature (reduced the duration
to 500ms to be closer to Android behaviour)

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

@@ -13,7 +13,7 @@ const double _kMaxIndicatorExtent = 64.0;
 const double _kMinIndicatorOpacity = 0.0;
 const double _kMaxIndicatorOpacity = 0.25;
 const Duration _kIndicatorHideDuration = const Duration(milliseconds: 200);
-const Duration _kIndicatorTimeoutDuration = const Duration(seconds: 1);
+const Duration _kIndicatorTimeoutDuration = const Duration(milliseconds: 500);
 final Tween<double> _kIndicatorOpacity = new Tween<double>(begin: 0.0, end: 0.3);
 
 class _Painter extends CustomPainter {
@@ -132,8 +132,11 @@ class _OverscrollIndicatorState extends State<OverscrollIndicator> {
   void _hide() {
     _hideTimer?.cancel();
     _hideTimer = null;
+    // Gaurding _hide() being called while indicator is already animating.
+    if (!_extentAnimation.isAnimating) {
       _extentAnimation.reverse();
     }
+  }
 
   void _updateState(ScrollableState scrollable) {
     if (scrollable.scrollBehavior is! ExtentScrollBehavior)
@@ -151,13 +154,16 @@ class _OverscrollIndicatorState extends State<OverscrollIndicator> {
 
   void _onScrollUpdated(ScrollableState scrollable) {
     final double value = scrollable.scrollOffset;
-    if ((value < _minScrollOffset || value > _maxScrollOffset) &&
-        ((value - _scrollOffset).abs() > kPixelScrollTolerance.distance)) {
-      _hideTimer?.cancel();
-      _hideTimer = new Timer(_kIndicatorTimeoutDuration, _hide);
+    if (_isOverscroll(value)) {
+      _refreshHideTimer();
+      // Hide the indicator as soon as user starts scrolling in the reverse direction of overscroll.
+      if (_isReverseScroll(value)) {
+        _hide();
+      } else {
         // Changing the animation's value causes an implicit setState().
         _extentAnimation.value = value < _minScrollOffset ? _minScrollOffset - value : value - _maxScrollOffset;
       }
+    }
     _updateState(scrollable);
   }
 
@@ -166,6 +172,21 @@ class _OverscrollIndicatorState extends State<OverscrollIndicator> {
     _hide();
   }
 
+  void _refreshHideTimer() {
+    _hideTimer?.cancel();
+    _hideTimer = new Timer(_kIndicatorTimeoutDuration, _hide);
+  }
+
+  bool _isOverscroll(double scrollOffset) {
+    return (scrollOffset < _minScrollOffset || scrollOffset > _maxScrollOffset) &&
+      ((scrollOffset - _scrollOffset).abs() > kPixelScrollTolerance.distance);
+  }
+
+  bool _isReverseScroll(double scrollOffset) {
+    final double delta = _scrollOffset - scrollOffset;
+    return scrollOffset < _minScrollOffset ? delta < 0 : delta > 0;
+  }
+
   bool _handleScrollNotification(ScrollNotification notification) {
     if (config.scrollableKey == null || config.scrollableKey == notification.scrollable.config.key) {
       final ScrollableState scrollable = notification.scrollable;