msglist [nfc]: Make ScrollToButton speed limit calculation more explicit in the code

Author: sirpengi

lib/widgets/message_list.dart

@@ -240,9 +240,8 @@ class ScrollToBottomButton extends StatelessWidget {
 
   Future<void> _navigateToBottom(BuildContext context) async {
     final distance = scrollController.position.pixels;
-    // Calculate time necessary if we're capping speed at 8pixels/ms (== 8000pixels/s)
-    final speedLimit = (distance / 8).ceil();
-    final durationMs = max(300, speedLimit);
+    final speedLimitMs = (1000 * distance / 8000).ceil();
+    final durationMs = max(300, speedLimitMs);
     scrollController.animateTo(
       0,
       duration: Duration(milliseconds: durationMs),