fix(cli): prevent ghost text wrapping infinite loop at narrow widths

[Anurag-M1]

Fixes #19985

Description

In packages/cli/src/ui/components/InputPrompt.tsx, getGhostTextLines attempts to wrap words using a while loop:

while (stringWidth(wordToProcess) > inputWidth) {
  let part = '';
  const wordCP = toCodePoints(wordToProcess);
  let partWidth = 0;
  let splitIndex = 0;
  for (let i = 0; i < wordCP.length; i++) {
    const char = wordCP[i];
    const charWidth = stringWidth(char);
    if (partWidth + charWidth > inputWidth) {
      break;
    }
    part += char;
    partWidth += charWidth;
    splitIndex = i + 1;
  }
  additionalLines.push(part);
  wordToProcess = cpSlice(wordToProcess, splitIndex);
}

When inputWidth is narrower than a single character's rendered width (such as wide CJK or emoji characters with width 2 in narrow terminals where inputWidth <= 1), splitIndex stays 0. Because wordToProcess = cpSlice(wordToProcess, splitIndex) fails to shrink, the loop spins indefinitely at 100% CPU and freezes the terminal prompt.

Solution

Added a safeguard after the character width loop:

if (splitIndex === 0 && wordCP.length > 0) {
  part = wordCP[0];
  splitIndex = 1;
}

This guarantees that wordToProcess shrinks by at least one code point per iteration, ensuring the hard-split loop always terminates.

Testing

• Added regression test in packages/cli/src/ui/components/InputPrompt.test.tsx under describe('ghost text wrapping') verifying that rendering with narrow inputWidth = 1 and wide character ghost text completes without hanging.
• All 200 unit tests in InputPrompt.test.tsx pass cleanly.
• npm run typecheck --workspace @google/gemini-cli passed with 0 errors.
• eslint passed with 0 warnings.

[github-actions]

📊 PR Size: size/S

• Lines changed: 42
• Additions: +42
• Deletions: -0
• Files changed: 2

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical issue where the CLI input prompt would enter an infinite loop and freeze the terminal when rendering ghost text in extremely narrow widths. By forcing the string processing loop to advance even when a character's width exceeds the available space, the fix ensures stability in constrained terminal environments.

Highlights

• Infinite Loop Prevention: Added a safety check in the ghost text wrapping logic to ensure that the processing loop always advances by at least one code point, preventing CPU hangs when the input width is narrower than a single character.
• Regression Testing: Introduced a new test case in InputPrompt.test.tsx that simulates narrow terminal widths with wide characters to verify the fix and prevent future regressions.

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[gemini-code-assist]

Code Review

This pull request fixes an infinite loop issue in InputPrompt.tsx that occurs when the inputWidth is narrower than a single codepoint (e.g., when dealing with wide characters). It introduces a guard to force-advance by one codepoint when splitIndex remains 0. Additionally, a regression test has been added in InputPrompt.test.tsx to verify this behavior and prevent future freezes. There are no review comments, so I have no feedback to provide.