UX: implements draft threads (#21361)

This commit implements all the necessary logic to create thread seamlessly. For this it relies on the same logic used for messages and generates a `staged-id`(using the format: `staged-thread-CHANNEL_ID-MESSAGE_ID` which is used to re-conciliate state client sides once the thread has been persisted on the backend.

Part of this change the client side is now always using real thread and channel objects instead of sometimes relying on a flat `threadId` or `channelId`.

This PR also brings three UX changes:
- thread starts from top
- number of buttons on message actions is dependent of the width of the enclosing container
- <kbd>shift + ArrowUp</kbd> will reply to the last message

plugins/chat/lib/chat/message_creator.rb

@@ -13,6 +13,7 @@ module Chat
       chat_channel:,
       in_reply_to_id: nil,
       thread_id: nil,
+      staged_thread_id: nil,
       user:,
       content:,
       staged_id: nil,
@@ -31,6 +32,7 @@ module Chat
       @incoming_chat_webhook = incoming_chat_webhook
       @upload_ids = upload_ids || []
       @thread_id = thread_id
+      @staged_thread_id = staged_thread_id
       @error = nil
 
       @chat_message =
@@ -57,7 +59,12 @@ module Chat
         create_thread
         @chat_message.attach_uploads(uploads)
         Chat::Draft.where(user_id: @user.id, chat_channel_id: @chat_channel.id).destroy_all
-        Chat::Publisher.publish_new!(@chat_channel, @chat_message, @staged_id)
+        Chat::Publisher.publish_new!(
+          @chat_channel,
+          @chat_message,
+          @staged_id,
+          staged_thread_id: @staged_thread_id,
+        )
         resolved_thread&.increment_replies_count_cache
         Jobs.enqueue(Jobs::Chat::ProcessMessage, { chat_message_id: @chat_message.id })
         Chat::Notifier.notify_new(chat_message: @chat_message, timestamp: @chat_message.created_at)
@@ -123,6 +130,8 @@ module Chat
     end
 
     def validate_existing_thread!
+      return if @staged_thread_id.present? && @thread_id.blank?
+
       return if @thread_id.blank?
       @existing_thread = Chat::Thread.find(@thread_id)
 
@@ -165,7 +174,7 @@ module Chat
 
     def create_thread
       return if @in_reply_to_id.blank?
-      return if @chat_message.in_thread?
+      return if @chat_message.in_thread? && !@staged_thread_id.present?
 
       if @original_message.thread
         thread = @original_message.thread
@@ -177,12 +186,15 @@ module Chat
             channel: @chat_message.chat_channel,
           )
         @chat_message.in_reply_to.thread_id = thread.id
-        Chat::Publisher.publish_thread_created!(
-          @chat_message.chat_channel,
-          @chat_message.in_reply_to,
-        )
       end
 
+      Chat::Publisher.publish_thread_created!(
+        @chat_message.chat_channel,
+        @chat_message.in_reply_to,
+        thread.id,
+        @staged_thread_id,
+      )
+
       @chat_message.thread_id = thread.id
 
       # NOTE: We intentionally do not try to correct thread IDs within the chain