Slash command completion erases last character of input in narrow terminals

Description

When the terminal window is narrower than the full set of slash command completion options, typing a slash command (e.g., /vbw:vibe) and pressing space causes the last character of the command to be visually erased when the completion dropdown appears. In this case, /vbw:vibe renders as /vbw:vib once the options are shown.

Steps to Reproduce

1. Resize terminal window to be narrower than the completion options dropdown (below ~80 columns)
2. Start typing a slash command with completion options, e.g. /vbw:vibe
3. Press space to trigger completion/options display
4. Observe: the last letter ("e" in "vibe") disappears from the input field when the options appear

Expected Behavior

The full command text (/vbw:vibe) should remain visible in the input field when completion options are displayed, regardless of terminal width.

Actual Behavior

The last character of the typed command is erased/overwritten when the completion dropdown renders in a narrow terminal.

Environment

• Claude Code version: 2.1.42
• Platform: macOS arm64 (Darwin 25.2.0)
• Terminal: Standard terminal, reproduced at widths below ~80 columns

Possibly Related Prior Fixes

• v2.1.39: "Fixed character loss at terminal screen boundary"
• v2.1.20: "Fixed wide character rendering artifacts where trailing columns were not cleared when replaced by narrower characters"
• v2.1.20: "Fixed ghost text flickering when typing slash commands mid-input"
• v2.1.31: "Narrow terminal layout for prompt footer"

This appears to be a remaining edge case in the same area — the completion dropdown rendering logic may be miscalculating the available width and truncating or overwriting the input field's trailing character.