[BUG] /tui fullscreen clips bottom rows under tmux status bar (doesn't account for multiplexer chrome)

Preflight

• [x] Searched existing issues — no duplicate found (closest are #48851, #49359, #51025, #51446, all different /tui fullscreen bugs)
• [x] Single bug report
• [x] Latest version

What's Wrong?

When Claude Code runs inside tmux and fullscreen rendering is active (/tui fullscreen), the TUI's drawable area assumes the full terminal window height and paints the bottom rows underneath the tmux status bar. The clipped rows include load-bearing UI: the shell-details footer ("Showing N lines of X", "← to go back · Esc/Enter/Space to close · x to stop"), thinking/working indicators, and the current task/phase line. They're invisible unless you shrink the font until the whole thing squeezes in above the tmux bar — which makes everything unreadably small.

What Should Happen?

The fullscreen layout should use the actual usable row count (terminal rows minus tmux status bar) as its canvas — or at minimum, the bottom chrome should remain visible / the content should be scrollable to reach the footer.

Steps to Reproduce

1. Launch tmux with a status bar at the bottom (default tmux config, or any tmux config with status-position bottom).
2. Inside tmux, open Ghostty (or any terminal) and start claude.
3. Run /tui fullscreen to enable fullscreen rendering.
4. Trigger any UI that extends near the bottom of the screen — e.g. open a long shell-details panel (or scroll through a long running background shell's output).
5. Observe: bottom row(s) of Claude Code's output are hidden behind the tmux bar. The footer line ("← to go back · Esc/Enter/Space to close · x to stop", "Showing N lines of X") is unreachable.
6. Workaround: Cmd+- to shrink font until everything fits above the tmux bar. Then it's all visible but tiny.

Version

2.1.116 (Claude Code)

Regression

I don't know

Platform

Anthropic API

OS

macOS (26.4.1)

Terminal

Other — Ghostty (inside tmux 3.5a, TERM=tmux-256color)

Additional

• Reproduced on Ghostty + tmux 3.5a, macOS 26.4.1.
• Screenshots available — will attach in a follow-up comment.
• Likely related: /tui fullscreen may be using the outer terminal row count directly instead of the tmux pane row count (which already excludes the status bar). Worth checking whether the fullscreen renderer is treating the alternate screen buffer as full-window rather than full-pane.