wip scroll: Go to end at first, and stay at end once there; TODO split?; TODO test

This is NFC as to the real message list, because so far the bottom
sliver there always has height 0, so that maxScrollExtent is 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 starting out scrolled to the end, and its
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].
@@ -311,6 +312,39 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
 
     return applyContentDimensions(effectiveMin, effectiveMax);
   }
+
+  bool get _isAtEnd => nearEqual(pixels, maxScrollExtent,
+    Tolerance.defaultTolerance.distance);
+
+  @override
+  bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
+    // Inspired by _TabBarScrollPosition.applyContentDimensions upstream.
+    bool changed = false;
+
+    final initial = !hasContentDimensions;
+    final wasAtEnd = hasContentDimensions && _isAtEnd;
+
+    if (!super.applyContentDimensions(minScrollExtent, maxScrollExtent)) {
+      changed = true;
+    }
+
+    if (initial) {
+      // The list is being laid out for the first time.
+      // Start out scrolled to the end.
+      // TODO what if the next _attemptLayout iteration finds more items below
+      //   and makes maxScrollExtent bigger?
+      correctPixels(this.maxScrollExtent);
+      changed = true;
+    } else if (wasAtEnd && !_isAtEnd) {
+      // 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(this.maxScrollExtent);
+      changed = true;
+    }
+
+    return !changed;
+  }
 }
 
 /// A version of [ScrollController] adapted for the Zulip message list.