- group writes when computing separators positions
- shows skeleton only on initial load
- forces date separator to be pinned when first message to prevent a pinned - not pinned - pinned sequence when loading more in past
- relies on `message.visible` property instead of checking `isElementInViewport`
- attempts to load next/prev messages earlier
- do not scroll to on fetch more
- hides `last visit` text while pinned
First follow-up to the feature introduced in #19034. We don't want to pollute main components like `chat-live-pane`, so we'll use a service to track and manage the state needed to display before-send warnings while composing a chat message.
We also move from acceptance tests to system specs.