Renderer paints cursor in wrong position and overprints existing text on every response (regression introduced in 2.1.163)

Summary

Starting in 2.1.163, the TUI miscalculates cursor position after rendering. Two linked symptoms on every response:

1. The cursor lands in the wrong row — visibly floating in empty space below the prompt.
2. Subsequent output is painted on top of already-rendered text, clobbering it (not just a stray cursor — real text overwrites real text).

This makes the session progressively unreadable as responses stack.

Regression

• 2.1.162: clean. Renders correctly, cursor tracks properly. (last good)
• 2.1.163: broken, as above. (first bad)
• Confirmed still broken through 2.1.179.

Reproduction

1. On 2.1.163+, send any prompt that produces a multi-line response.
2. Observe the cursor settle in the wrong row after the response renders.
3. Send another prompt — new output overprints the previous response's text.

Content-independent: reproduces with plain prose as well as markdown (tables / code blocks / emoji). It is not tied to wide characters or any specific markdown construct — every response drifts.

What I ruled out

• Plugins / hooks / statusline / custom skills — reproduces under --safe-mode.
• Terminal multiplexer — reproduces both inside tmux 3.5a and in a bare terminal (no tmux).
• Markdown / emoji width — reproduces on plain-text responses too.

Environment

• OS: macOS 25.5.0 (Darwin 25.5.0), arm64 (Apple Silicon)
• Claude Code: first bad 2.1.163, last good 2.1.162, native install
• Terminal: tmux 3.5a (TERM=tmux-256color) and bare terminal both affected
• COLORTERM=truecolor

Workaround

Downgrade to 2.1.162: repoint the native shim (~/.local/bin/claude → versions/2.1.162) with autoupdate disabled.