[Bug] WeChat IME: Moving cursor forward then typing Chinese replaces following text

[kshq1996]

Issue Description

Using WeChat Input Method (微信输入法) on macOS, when typing Chinese text in Ghostty and moving the cursor forward before continuing to type, the newly committed Chinese characters occasionally overwrite/replace the existing text after the cursor, instead of being inserted between existing characters. Pure ASCII/English input works normally and does not exhibit this behavior.

Expected Behavior

Newly committed Chinese characters should be inserted at the cursor position without affecting the existing text after the cursor.

Actual Behavior

Newly committed Chinese characters overwrite/replace the existing text that follows the cursor. This occurs intermittently, not on every input.

Reproduction Steps

1. Open Ghostty (macOS).
2. Switch to WeChat Input Method (Chinese Pinyin).
3. Type some Chinese text, e.g., 你好世界.
4. Move the cursor forward (e.g., press the left arrow key) to position it between existing characters.
5. Continue typing Chinese characters (e.g., 测试).
6. Observe: The newly committed Chinese characters sometimes replace the text that follows the cursor, rather than being inserted before it.

Ghostty Logs

No response

Ghostty Version

Ghostty 1.3.1

Version
  - version: 1.3.1
  - channel: stable
Build Config
  - Zig version   : 0.15.2
  - build mode    : .ReleaseFast
  - app runtime   : .none
  - font engine   : .coretext
  - renderer      : renderer.generic.Renderer(renderer.Metal)
  - libxev        : kqueue

OS Version Information

macOS 27.2

(Linux only) Display Server

None

(Linux only) Desktop Environment/Window Manager

No response

Minimal Ghostty Configuration

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Additional Relevant Configuration

No response

I acknowledge that:

• I have reviewed the FAQ and confirm that my issue is NOT among them.
• I have searched the Ghostty documentation and confirm there doesn't exist a configuration option that solves my issue.
• I have searched the Ghostty repository (both open and closed Discussions and Issues) and confirm this is not a duplicate of an existing issue or discussion.
• I have checked the "Preview" tab on all text fields to ensure that everything looks right, and have wrapped all configuration and code in code blocks with a group of three backticks (```) on separate lines.

[xinshengfan]

Same symptom here — I think I have the root cause: it's the WeChat IME's
"auto space between CJK and Latin" feature, and what actually lands in the
PTY is a spurious forward-delete (CSI 3~).

Environment: Ghostty 1.4.0 (stable, arm64), macOS <x.y.z>,
WeChat IME (WeType.app) with 中文与英文/数字之间自动加空格 = ON.

Repro: type 12345678, press ← 4x, commit 在 twice, then type a Latin
letter. Characters right of the cursor disappear; the program's buffer loses
them (not a rendering artifact).

Evidence — I ran a raw-stdin logger inside Ghostty (setRawMode + hex dump
of every chunk). Relevant lines:

1b 5b 44            ← (x4, cursor moves left)
e5 9c a8 20         "在 "  one commit  (auto space)
1b 5b 44            ←
20 64               " d"   one commit  (auto space)
1b 5b 33 7e         CSI 3~  ← 32 ms later, never pressed
64                  "d"
1b 5b 33 7e         CSI 3~  ← 27 ms later
final buffer: "1234dd在在在 dd678"   (the '5' is gone)

A physical Backspace from the same session arrives as bare 0x7F, so the two
CSI 3~ are a different source: they are IME-synthesized.

Where it goes wrong (checked against main today):

• macOS keycode 117 maps to .delete (src/input/keycodes.zig:322), and
  src/apprt/embedded.zig:121 resolves the key identity from the native
  keycode, not from the event's text.
• SurfaceView_AppKit.swift:2100 (insertText(_:replacementRange:)) ignores
  replacementRange and funnels committed text through committedTextAction
  as key events; AppKit-native text views resolve the same event via
  characters (\x7F) and treat it as deleteBackward or ignore it — which
  is why only Ghostty eats the following character.
• The existing guard shouldSuppressComposingControlInput
  (SurfaceView_AppKit.swift:2182) only fires while composing == true and
  only for < 0x20, so it cannot catch this: the delete comes after commit
  and carries 0x7F.

Extra data point: replaying the byte log, the first CSI 3~ deleted a
leftover auto-space (invisible), the second deleted a real character. So the
discriminator for "sometimes it happens" is simply: is the character right of
the cursor a space? Typing a space first hides the bug because the IME eats
that space instead.

Suggested fix: drop key events whose text is a control character
(< 0x20 and 0x7F) even when composing == false, or ignore IME-synthesized
key events that carry no real hardware press.

Workaround: turn off 中文与英文/数字之间自动加空格, or use an IME without it
(Baidu Wubi is unaffected). Same operation is fine in iTerm2 / TextEdit /
Sublime / Tabby.