Avoid divide by zero in scroll thumb rectangle calculations (#10271)

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

@@ -156,8 +156,13 @@ class _ScrollbarPainter extends ChangeNotifier implements CustomPainter {
 
   void _paintThumb(double before, double inside, double after, double viewport, Canvas canvas, Size size,
                    void painter(Canvas canvas, Size size, double thumbOffset, double thumbExtent)) {
-    final double thumbExtent = math.max(math.min(viewport, _kMinThumbExtent), viewport * inside / (before + inside + after));
-    final double thumbOffset = before * (viewport - thumbExtent) / (before + after);
+    double thumbExtent = math.min(viewport, _kMinThumbExtent);
+    if (before + inside + after > 0.0)
+      thumbExtent = math.max(thumbExtent, viewport * inside / (before + inside + after));
+
+    final double thumbOffset = (before + after > 0.0) ?
+        before * (viewport - thumbExtent) / (before + after) : 0.0;
+
     painter(canvas, size, thumbOffset, thumbExtent);
   }
 

packages/flutter/test/material/scrollbar_test.dart

@@ -6,6 +6,17 @@ import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+class TestCanvas implements Canvas {
+  TestCanvas([this.invocations]);
+
+  final List<Invocation> invocations;
+
+  @override
+  void noSuchMethod(Invocation invocation) {
+    invocations?.add(invocation);
+  }
+}
+
 void main() {
   testWidgets('Scrollbar doesn\'t show when tapping list', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -48,4 +59,39 @@ void main() {
     await tester.pump(const Duration(milliseconds: 200));
     await tester.pump(const Duration(milliseconds: 200));
   });
+
+  testWidgets('ScrollbarPainter does not divide by zero', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Container(
+        height: 200.0,
+        width: 300.0,
+        child: new Scrollbar(
+          child: new ListView(
+            children: <Widget>[
+              new Container(height: 40.0, child: const Text('0')),
+            ]
+          )
+        )
+      )
+    );
+
+    final CustomPaint custom = tester.widget(find.descendant(of: find.byType(Scrollbar), matching: find.byType(CustomPaint)).first);
+    final dynamic scrollPainter = custom.foregroundPainter;
+    final ScrollMetrics metrics = new FixedScrollMetrics(
+      minScrollExtent: 0.0,
+      maxScrollExtent: 0.0,
+      pixels: 0.0,
+      viewportDimension: 100.0,
+      axisDirection: AxisDirection.down
+    );
+    scrollPainter.update(metrics, AxisDirection.down);
+    await tester.pump(const Duration(milliseconds: 200));
+    await tester.pump(const Duration(milliseconds: 200));
+
+    final List<Invocation> invocations = <Invocation>[];
+    final TestCanvas canvas = new TestCanvas(invocations);
+    scrollPainter.paint(canvas, const Size(10.0, 100.0));
+    final Rect thumbRect = invocations.single.positionalArguments[0];
+    expect(thumbRect.isFinite, isTrue);
+  });
 }