scroll [nfc]: Introduce ScrollToEndActivity

We'll use this to customize the behavior.

It also makes a handy marker in tests, when using the testing-only
[ScrollPosition.activity] getter for inspecting what's going on in
the scroll behavior.

Author: gnprice

lib/widgets/scrolling.dart

@@ -43,6 +43,21 @@ class _SingleChildScrollViewWithScrollbarState
   }
 }
 
+/// An activity that animates a scroll view smoothly to its end.
+///
+/// In particular this drives the "scroll to bottom" button
+/// in the Zulip message list.
+class ScrollToEndActivity extends DrivenScrollActivity {
+  ScrollToEndActivity(
+    super.delegate, {
+    required super.from,
+    required super.to,
+    required super.duration,
+    required super.curve,
+    required super.vsync,
+  });
+}
+
 /// A version of [ScrollPosition] adapted for the Zulip message list,
 /// used by [MessageListScrollController].
 class MessageListScrollPosition extends ScrollPositionWithSingleContext {
@@ -217,7 +232,7 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
     final durationSec = math.max(durationSecAtSpeedLimit,
       minDuration.inMilliseconds / 1000.0);
     final duration = Duration(milliseconds: (durationSec * 1000.0).ceil());
-    beginActivity(DrivenScrollActivity(this, vsync: context.vsync,
+    beginActivity(ScrollToEndActivity(this, vsync: context.vsync,
       from: pixels, to: target, duration: duration, curve: Curves.linear));
   }
 }

test/widgets/scrolling_test.dart

@@ -204,7 +204,7 @@ void main() {
         position.scrollToEnd();
         await tester.pump();
         check(position.extentAfter).equals(300);
-        check(position.activity).isA<DrivenScrollActivity>();
+        check(position.activity).isA<ScrollToEndActivity>();
 
         // The scrolling moves at a stately pace; …
         await tester.pump(Duration(milliseconds: 100));
@@ -236,7 +236,7 @@ void main() {
         // Start scrolling to end.
         position.scrollToEnd();
         await tester.pump();
-        check(position.activity).isA<DrivenScrollActivity>();
+        check(position.activity).isA<ScrollToEndActivity>();
 
         // Let it scroll, plotting the trajectory.
         final log = <double>[];