[FEATURE]: TUI scroll buffer should not truncate old messages — breaks fork from history

[and0rl]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

In long sessions, the TUI scroll buffer truncates older messages — scrolling up
reaches a hard limit before the beginning of the conversation. This makes it
impossible to:

1. Review the full history of a session directly in the TUI
2. Click on an early message to fork the session from that point

The fork-from-message feature exists and works, but it's only accessible by
clicking on a visible message. If the message is outside the scroll buffer,
it's effectively unreachable from the UI.

Current workaround

The only workaround is to use opencode session cat <id> or read the raw JSON
file to find the message, then manually edit the session file to create a fork.
This is cumbersome and not user-friendly.

Proposed Solution

One or more of the following:

• Remove or significantly increase the TUI scroll buffer limit, keeping the
  full session history scrollable
• Add a "jump to message" or search within session feature in the TUI
• Add a opencode session fork <session-id> <message-id> CLI command
  so forks can be created without needing to scroll to the message visually

Expected behavior

Users should be able to scroll to any message in the session history and
fork from it, regardless of session length.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Paginated message loading for long sessions #6548: Paginated message loading for long sessions — closely related, covers bounded message windows and loading older history on demand in the TUI
• [FEATURE]: TUI - Search for and find string in session buffer #4714: TUI - Search for and find string in session buffer — overlaps with the "search within session" part of your proposed solution