Thai SARA AM (U+0E33) width mismatch causing erratic line jumping and output duplication (spamming) in tmux

[billnitiup]

What happened?

When using Gemini CLI inside a tmux environment, any Thai text containing the character SARA AM (U+0E33) triggers severe rendering bugs that make
the CLI unusable.

Symptoms identified:

1. Output Duplication (Streaming Spam): When the AI streams a response containing SARA AM, the terminal fails to refresh the current line
   correctly. Instead, it "spams" the same line multiple times vertically. (e.g., a single bullet point appearing 8-10 times, as shown in the
   attached example).
2. Self-Jumping Input Line: During interactive typing, entering SARA AM causes the current input line to "jump" downward spontaneously, leaving
   large vertical gaps between the prompt and the previous output.
3. Cursor Desync: The internal layout engine (Ink) loses track of the actual cursor position because it perceives SARA AM as part of a 1-column
   cluster, while tmux/terminal renders it as a 2-column sequence.

The Technical Root Cause:

• Ink's behavior: Uses Intl.Segmenter which merges Consonant + SARA AM into a single grapheme cluster of width 1.
• tmux/xterm.js behavior: Renders the same sequence across 2 columns (matching standard wcwidth logic).
• The Mismatch: Since the terminal's physical cursor is 1 unit ahead of where Ink thinks it is, every frame refresh results in Ink sending "move
  cursor" or "clear line" commands to incorrect coordinates. This triggers tmux to scroll or wrap lines prematurely to protect the buffer, leading
  to the "spamming" effect.

What did you expect to happen?

The CLI should maintain a 1:1 coordinate sync with the terminal grid. Typing or streaming SARA AM should result in a single, stable line of text
without triggering unintended line breaks or duplicated output.

Client information

Client Information

Run gemini to enter the interactive CLI, then run the /about command.

> /about
╭───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│                                                                                                                                                   │
│ About Gemini CLI                                                                                                                                  │
│                                                                                                                                                   │
│ CLI Version                                        0.37.2                                                                                         │
│ Git Commit                                         545e956c3                                                                                      │
│ Model                                              gemini-3-flash-preview                                                                         │
│ Sandbox                                            no sandbox                                                                                     │
│ OS                                                 linux                                                                                          │
│ Auth Method                                        Signed in with Google                                             │
│ Tier                                               Gemini Code Assist in Google One AI Pro                                                        │
│                                                                     │
│                                                                                                                                                   │
╰───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯

Login information

Logged in via Google Account.

Anything else we need to know?

• Platform: Linux (Debian) + tmux.
• Consistency: This issue is exclusive to SARA AM (U+0E33). Other Thai combining marks (vowels/tones) work correctly because both Ink and
  terminals agree they have 0 width.
• Replicability: The issue is highly replicable by simply asking the AI to "Response in Thai with words like 'กำลัง', 'ทำ', 'แนะนำ'".

Visual Evidence of the "Spam" bug:

มันทำอะไรได้บ้าง? (The God Mode)
มันทำอะไรได้บ้าง? (The God Mode)
มันทำอะไรได้บ้าง? (The God Mode)
มันทำอะไรได้บ้าง? (The God Mode)
... (repeated multiple times for a single output line)

Proposed Fix:
Update the string-width calculation within the Ink library to treat U+0E33 as having a visual width of 1, effectively making the Consonant + SARA
AM cluster have a total width of 2, aligning it with the behavior of terminal grids.

[Adib234]

Thank you for this extremely detailed report and for diagnosing the root cause involving the SARA AM (U+0E33) width mismatch between Ink and tmux. Since you have already identified a potential fix, we are labeling this as 'help wanted' and would welcome a pull request to address this rendering behavior.