I've setup the SlackNotifier in my GCP build pipeline, everything is working pretty well but when messages come through in the Slack desktop client for Windows, the notification popup says "no preview available" even though the actual message in Slack appears fine.


This seems to be due to the fact that in the WebhookMessage post, the "Text" property is not included and Windows / Slack (on Windows) not supporting rich-text content. Slack documentation suggests including the "Text" property in the payload which will be used as fallback when rich-text is not supported.
More details are available in the StackOverfow issue I posted: https://stackoverflow.com/questions/76657027/google-cloud-build-slack-notifier-message-no-preview-available
The posted answer explains the (suspected) problem and solution and he even created a fork with suggested code changes - onix@e83dab3