Do not override incoming webhook channel for mattermost and slack

app/models/project_services/chat_notification_service.rb

@@ -49,10 +49,12 @@ class ChatNotificationService < Service
     return false unless message
-    opt = {}
-    opt[:channel]  = get_channel_field(object_kind).presence || channel || default_channel
+    channel_name   = get_channel_field(object_kind).presence || channel
+    opt            = {}
+    opt[:channel]  = channel_name if channel_name
     opt[:username] = username if username
+
     notifier = Slack::Notifier.new(webhook, opt)
     notifier.ping(message.pretext, attachments: message.attachments, fallback: message.fallback)
@@ -71,7 +73,7 @@ class ChatNotificationService < Service
     fields.reject { |field| field[:name].end_with?('channel') }
   end
-  def default_channel
+  def default_channel_placeholder
     raise NotImplementedError
   end
@@ -103,7 +105,7 @@ class ChatNotificationService < Service
   def build_event_channels
     supported_events.reduce([]) do |channels, event|
-      channels << { type: 'text', name: event_channel_name(event), placeholder: default_channel }
+      channels << { type: 'text', name: event_channel_name(event), placeholder: default_channel_placeholder }
     end
   end

app/models/project_services/mattermost_service.rb

@@ -35,7 +35,7 @@ class MattermostService < ChatNotificationService
     ]
   end
-  def default_channel
+  def default_channel_placeholder
     "#town-square"
   end
 end

app/models/project_services/slack_service.rb

@@ -34,7 +34,7 @@ class SlackService < ChatNotificationService
     ]
   end
-  def default_channel
+  def default_channel_placeholder
     "#general"
   end
 end

changelogs/unreleased/issue_25887.yml

+---
+title: Do not override incoming webhook for mattermost and slack
+merge_request:
+author:

spec/support/slack_mattermost_notifications_shared_examples.rb

@@ -105,7 +105,7 @@ RSpec.shared_examples 'slack or mattermost notifications' do
       allow(chat_service).to receive(:username).and_return(username)
       expect(Slack::Notifier).to receive(:new).
-       with(webhook_url, username: username, channel: chat_service.default_channel).
+       with(webhook_url, username: username).
        and_return(
          double(:slack_service).as_null_object
        )