[vim/vim] popup: opacity popup over a terminal is not cleared after closed (Issue #20846)

mityu created an issue (vim/vim#20846)

### Steps to reproduce


1. Make this `vimrc.vim`.
    ```vim
    set nocompatible

    function s:repro() abort
      terminal ++curwin

      " Maybe this is an timing issue.  Rarely this bug doesn't happen without
      " this.
      " Probably... we have to wait until the invoked shell fully starts up; i.e.
      " we should wait for the state that Vim has no more need to redraw the
      " terminal window...
      sleep 100m

      let popupid = popup_create('ABC', #{ opacity: 30 })
      call timer_start(100, {-> popup_close(popupid)})
    endfunction

    command! Repro call s:repro()
    ```
2. Open Vim: `vim -u /path/to/vimrc.vim`
3. Type `:Repro<CR>` to execute the `:Repro` command.
4. Check the screen


### Expected behaviour

- **Expected behavior**

No popup windows appear on screen.

<img width="901" height="546" alt="Image" src="https://github.com/user-attachments/assets/52206191-0fea-4737-9cb7-4dd292f0b674" />

- **Actual behavior**

Popup is still on screen.

<img width="901" height="514" alt="Image" src="https://github.com/user-attachments/assets/7c69f5f7-0635-4ba9-949c-b63ca6b7cdee" />


### Version of Vim

Vim 9.2.0856

### Environment

OS: NixOS

```
$ nix-info -m
 - system: `"x86_64-linux"`
 - host os: `Linux 6.18.39, NixOS, 26.11 (Zokor), 26.11.20260723.e2587ca`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.34.8`
 - channels(root): `"nixos-25.11"`
 - nixpkgs: `/nix/store/llgwlxshmy0ifvxh7f8wq53vk5x7vd13-source`
```

Terminal:

- WezTerm: `wezterm 0-unstable-2026-07-16`
- Ghostty: `Ghostty 1.3.1`
- XTerm: `XTerm(410)`

Shell:

- bash: `GNU bash, version 5.3.15(1)-release (x86_64-pc-linux-gnu)`
- fish: `fish, version 4.8.1`


### Logs and stack traces

```shell

```

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