Open chat history at a pinned message or reply, then scroll to the present (GRYT-1686)

[sivert-io]

The client half of opening chat history at any message. Task: GRYT-1686. The server half is server#282. Without it, this falls back to the 1,000-message search from #811, so it can merge first.

What it does

• Clicking an old pin or reply quote asks the server for the page around that message and opens history there, with the message in view.
• Scrolling down loads newer pages. When they reach the present, the window merges into the channel and the bar goes away.
• While you're in a window, a bar above the composer says "You're looking at older messages", with a Jump to present button.
• Sending a message, or switching conversation, also returns to the present.

How

• useHistoryWindow keeps the window as its own list on top of the channel's. useChat re-syncs the channel list from a cache that always runs up to the newest message, so a list with a gap can't live in it.
  • Edits, reactions, deletions and settled attachments reach the window through the same handlers.
  • New messages don't, because they belong at the present.
• useChat leaves window pages (around, after, or before while a window is open) to the window, and shows the window's list, older-page loader and hasOlder while it's open.
• useChatScroll loads newer pages within 200px of the bottom of a window. Inside a window it no longer auto-scrolls on a changed last row, and the ResizeObserver stops holding the bottom.
• useJumpToMessage tries openAt first. It falls back to paging older history when the answer is unsupported, and says the message is gone when it's missing.

Checked

In a headless browser against a seeded channel (220 messages, a pin on the 4th):

• server#282: the click shows the pinned message in view after about 1.7 s, with the bar up. Four newer pages reach the present, and the window merges and drops the bar.
• Jump to present: back at the bottom with only the latest page loaded. At first it stopped about 900px short, which is why it now scrolls instantly.
• Main's server (no around): falls back to paging, and the message is in view after about 4 s.
• No page errors in any run. yarn lint passes.

Look at

• useChat changes are small but sit on the path every channel uses. Nothing changes until openAt is called: historyWindow.messages is null, and every branch falls through to the old values.
• MLS DMs don't use the window yet, because their timeline merges an archive (mergeTimeline). They get the paging search.
• An older server answers around with the newest page. If a normal latest-page fetch, from a reconnect for instance, lands while a jump is waiting, the jump reads it as "unsupported" and pages instead. It still finds the message, just more slowly.

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