Focus on app icons are stolen in a few seconds after first keyboard navigation

### What Happened?

Regression of GTK 4 porting.
Originally reported at https://github.com/elementary/applications-menu/pull/710#issue-5101257054.

Better to watch the screencast:

https://github.com/user-attachments/assets/2c8f76d1-19ab-423c-ba4a-83a457f114fb

But this also happens with other indicators, so I suppose this would be an issue of Wingpanel itself:

https://github.com/user-attachments/assets/61a3b55b-3039-407c-a204-24f596300bfd

### Steps to Reproduce

1. Open Applications Menu
2. Focus on the search entry in Grid View
3. Press ↓ to move focus to the app icons
4. Wait for a few seconds
5. See the focus are stolen i.e. the blue indication that has focus disappears

### Expected Behavior

Focus on app icons should not be stolen as before in GTK 3 version of Applications Menu.

### OS Version

Early Access

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Early Access or Compiled from git

### Log Output

```shell

```

### Hardware Info

_No response_

Start by reproducing the issue in Applications Menu's Grid View on the Wayland session, following the listed keyboard-navigation steps, then inspect the GTK 4 focus handling and compare it with the prior GTK 3 behavior. Done means focus remains visibly on the app icons after waiting several seconds and the behavior also works for the affected indicators in Wingpanel.

Written by the indexing model from the issue text.