scroll: Stay at end once there

This is NFC as to the real message list, because so far the bottom
sliver there always has height 0, so that both maxScrollExtent and
this.maxScrollExtent are always 0.

This is a step toward letting us move part of the message list into
the bottom sliver, because it means that doing so would preserve the
list's current behavior of remaining scrolled to the end once there
as e.g. new messages arrive.

Author: gnprice

lib/widgets/scrolling.dart

@@ -2,6 +2,7 @@ import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/physics.dart';
 import 'package:flutter/rendering.dart';
 
 /// A [SingleChildScrollView] that always shows a Material [Scrollbar].
@@ -320,6 +321,9 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
     return applyContentDimensions(effectiveMin, effectiveMax);
   }
 
+  bool _nearEqual(double a, double b) =>
+    nearEqual(a, b, Tolerance.defaultTolerance.distance);
+
   bool _hasEverCompletedLayout = false;
 
   @override
@@ -337,6 +341,13 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
         correctPixels(target);
         changed = true;
       }
+    } else if (_nearEqual(pixels, this.maxScrollExtent)
+        && !_nearEqual(pixels, maxScrollExtent)) {
+      // The list was scrolled to the end before this layout round.
+      // Make sure it stays at the end.
+      // (For example, show the new message that just arrived.)
+      correctPixels(maxScrollExtent);
+      changed = true;
     }
 
     // This step must come after the first-time correction above.
@@ -345,6 +356,11 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
     // then the base implementation of [applyContentDimensions] would
     // bring it in bounds via a scrolling animation, which isn't right when
     // starting from the meaningless initial 0.0 value.
+    //
+    // For the "stays at the end" correction, it's not clear if the order
+    // matters in practice.  But the doc on [applyNewDimensions], called by
+    // the base [applyContentDimensions], says it should come after any
+    // calls to [correctPixels]; so OK, do this after the [correctPixels].
     if (!super.applyContentDimensions(minScrollExtent, maxScrollExtent)) {
       changed = true;
     }

test/widgets/scrolling_test.dart

@@ -243,6 +243,35 @@ void main() {
         .bottom.equals(600);
     });
 
+    testWidgets('stick to end of list when it grows', (tester) async {
+      final controller = MessageListScrollController();
+      await prepare(tester, controller: controller,
+        topHeight: 400, bottomHeight: 400);
+      check(tester.getRect(findBottom))..top.equals(200)..bottom.equals(600);
+
+      // Bottom sliver grows; remain scrolled to (new) bottom.
+      await prepare(tester, controller: controller,
+        topHeight: 400, bottomHeight: 500);
+      check(tester.getRect(findBottom))..top.equals(100)..bottom.equals(600);
+    });
+
+    testWidgets('when not at end, let it grow without following', (tester) async {
+      final controller = MessageListScrollController();
+      await prepare(tester, controller: controller,
+        topHeight: 400, bottomHeight: 400);
+      check(tester.getRect(findBottom))..top.equals(200)..bottom.equals(600);
+
+      // Scroll up (by dragging down) to detach from end of list.
+      await tester.drag(findBottom, Offset(0, 100));
+      await tester.pump();
+      check(tester.getRect(findBottom))..top.equals(300)..bottom.equals(700);
+
+      // Bottom sliver grows; remain at existing position, now farther from end.
+      await prepare(tester, controller: controller,
+        topHeight: 400, bottomHeight: 500);
+      check(tester.getRect(findBottom))..top.equals(300)..bottom.equals(800);
+    });
+
     testWidgets('position preserved when scrollable rebuilds', (tester) async {
       // Tests that [MessageListScrollPosition.absorb] does its job.
       //