[vim/vim] GTK3 UI has font rendering issues (Issue #21063)

MeanEYE created an issue (vim/vim#21063)

### Steps to reproduce

1. Open existing document;
2. Press `ciw` and retype the same text;
3. Cursor leaves rendering artefacts.

<img width="361" height="223" alt="Image" src="https://github.com/user-attachments/assets/8323cbf3-580c-44fe-8420-007f4c8a2eb4" />

Happens regardless of configured colorscheme.

https://github.com/user-attachments/assets/59e25558-153e-4132-b892-b2569a2eec93

### Expected behaviour

There shouldn't be any artifacts. When writing this text on a new, empty, line issue doesn't occur, only when replacing text.

### Version of Vim

9.2 Included patches: 1-858

### Environment

Debian GNU/Linux forky/sid (testing, up to date)
Vim is not ran in terminal or shell so probably irrelevant. Gnome-Shell, latest.

### Logs and stack traces

```shell

```

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