[Bug] Classic TUI renderer corrupts text on repaint with duplicated lines and dropped characters

Bug Description
Title: Classic (tui: "default") renderer corrupts text on repaint — duplicated lines with dropped characters

Summary: After switching from tui: "fullscreen" back to tui: "default", the terminal output becomes garbled: previously rendered lines are partially redrawn over themselves, producing doubled lines with missing/scrambled characters.

Symptoms (see screenshot):

• Lines appear twice, the second copy with characters dropped or mangled: "fullscreen" → "creen", "selection" → "selaction"/"selation", "reverting" → "rev rting", "2.1.196" → "2.1 96", "Darwin 25.4.0" → "Darwin 25.4 0".
• Stray characters land in input echo too: "muchkmuch better", "thanktyou very mush".
• Looks like incremental redraw isn't clearing prior cell contents before writing new ones.

When it happens: Started right after relaunching into tui: "default" from tui: "fullscreen" mid-session. Scrolling/redrawing the scrollback shows the corruption.

Environment: Claude Code 2.1.196, macOS (Darwin 25.4.0), zsh. (Terminal emulator: ___ — fill in, e.g. iTerm2 / Terminal.app / Ghostty / WezTip.)

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 2.1.196
• Feedback ID: 51cf127a-898c-49be-b41c-0997bf9d99b6

Errors

[]