Update to set a value for the gap between message groups (#625)

lib/src/util.dart

@@ -111,6 +111,7 @@ List<Object> calculateChatMessages(
   bool dateIsUtc = false,
   String? dateLocale,
   required int groupMessagesThreshold,
+  double messagesSpacerHeight = 12,
   String? lastReadMessageId,
   required bool showUserNames,
   DateFormat? timeFormat,
@@ -217,7 +218,7 @@ List<Object> calculateChatMessages(
       chatMessages.insert(
         0,
         MessageSpacer(
-          height: 12,
+          height: messagesSpacerHeight,
           id: message.id,
         ),
       );

lib/src/widgets/chat.dart

@@ -55,6 +55,7 @@ class Chat extends StatefulWidget {
     this.emptyState,
     this.fileMessageBuilder,
     this.groupMessagesThreshold = 60000,
+    this.messagesSpacerHeight = 12,
     this.hideBackgroundOnEmojiMessages = true,
     this.imageGalleryOptions = const ImageGalleryOptions(
       maxScale: PhotoViewComputedScale.covered,
@@ -176,6 +177,10 @@ class Chat extends StatefulWidget {
   /// is lower than this threshold, they will be visually grouped.
   final int groupMessagesThreshold;
 
+  /// Height value of spacers added to separate message groups.
+  /// Default value is 12. If 0, no gap between groups.
+  final double messagesSpacerHeight;
+
   /// See [Message.hideBackgroundOnEmojiMessages].
   final bool hideBackgroundOnEmojiMessages;
 
@@ -594,6 +599,7 @@ class ChatState extends State<Chat> {
         lastReadMessageId: widget.scrollToUnreadOptions.lastReadMessageId,
         showUserNames: widget.showUserNames,
         timeFormat: widget.timeFormat,
+        messagesSpacerHeight: widget.messagesSpacerHeight,
       );
 
       _chatMessages = result[0] as List<Object>;