[FEATURE]:Show session scrollbar by default for long chats

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

The session scrollbar is hidden by default, which makes it difficult to discover and navigate long conversations, especially for users relying on a visible scrollbar and drag handle.

Proposed change

Show the session scrollbar by default in the TUI for long chats.

The scrollbar should remain hidden when the session content does not require scrolling.

Expected behavior

• Long sessions show a visible scrollbar by default.
• Short sessions can continue to hide the scrollbar.
• Existing scrollbar functionality remains unchanged.
• The change should not affect keyboard-based scrolling.

Related PR

This issue is addressed by the pull request that implements the change.