macOS: check for updates freezes and glitches terminal output

[rrotter]

Issue Description

On macOS, "Check for updates…" causes the terminal to freeze until the dialog is cleared, and also causes temporary graphics corruption if there is text scrolling in the terminal window directly behind the dialog.

Expected Behavior

"Check for updates" dialog appears, and any active terminal windows continue rendering as normal, or at least freeze cleanly with no artifacts.

Actual Behavior

"Check for updates" dialog appears, all terminal output freezes, text scroll in window behind dialog is corrupted until dialog is closed.

Reproduction Steps

1. open ghostty
2. move window to center near of screen, where "Check for updates" dialog normally appears
3. start a process with fast scrolling text, such as a long running CI or compile job
4. Click "Ghostty" menu > "Check for Updates…"
5. all text stops moving, but not quite simultaneously, leaving screen tearing like artifacts arount the dialog
6. drag the dialog around the screen to produce additional artifacts

Ghostty Logs

No response

Ghostty Version

~ % ghostty +version
Ghostty 1.2.3

Version
  - version: 1.2.3
  - channel: stable
Build Config
  - Zig version   : 0.14.1
  - build mode    : builtin.OptimizeMode.ReleaseFast
  - app runtime   : apprt.Runtime.none
  - font engine   : font.main.Backend.coretext
  - renderer      : renderer.generic.Renderer(renderer.Metal)
  - libxev        : kqueue

### OS Version Information

macOS Sequoia 15.6.1 (24G90)

### (Linux only) Display Server

None

### (Linux only) Desktop Environment/Window Manager

_No response_

### Minimal Ghostty Configuration

```ini
# works exactly the same with no config file

### Additional Relevant Configuration

_No response_

### I acknowledge that:

- [X] I have reviewed the FAQ and confirm that my issue is NOT among them.
- [X] 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.
- [X] 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.

[mitchellh]

I'm going to close this as outdated because our check for updates dialog is gone in main.