[Bug] Fullscreen TUI mode missing draggable scrollbar and position indicator

Bug Description
▎ /tui fullscreen has no draggable scrollbar; /tui default does
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▎ In fullscreen mode the transcript can only be scrolled with the keyboard/wheel. There is no visible scrollbar track to grab and drag, which the default renderer provides.
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▎ Why it matters. In long sessions the transcript is thousands of lines. A draggable scrollbar is the only way to move a long distance in one gesture and, just as importantly, the only position indicator — without it there is no way to tell how far back in the session you are, or how much is above you.
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▎ Expected: fullscreen keeps the same grab-and-drag scrollbar as /tui default.
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▎ Environment: Ubuntu, GNOME on Wayland, Claude Code CLI in a terminal.

Environment Info

• Platform: linux
• Terminal: gnome-terminal
• Version: 2.1.238
• Feedback ID: 1203d2bf-f9a7-4284-a04a-32750568edf0

Errors

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