Small blank window briefly appears when initially running game in fullscreen

### Tested versions

- Reproduceable in v4.5.stable.official [876b29033]

### System information

MacOS 15.6.1 (24G90); v4.5.stable.official [876b29033]; Forward+

### Issue description

When running a project in fullscreen by default, a small blank window appears atop of the normal loading background. This did not happen in previous versions.

### Steps to reproduce

To reproduce:

1. Create a new project with default settings
2. Create a new main scene for the project within res://
3. Go the Project Settings > General > Display > Window > Mode and set this to 'Fullscreen'
4. Close Project Settings
5. Run the project; when you are asked to define a main scene, click 'Select Current'
6. The project will begin to run; whilst doing so, a small window will briefly appear atop the fullscreen background.

### Minimal reproduction project (MRP)

Barebones project:
[new-game-project.zip](https://github.com/user-attachments/files/22355474/new-game-project.zip)

Example of this in action:

Start with the attached minimal reproduction project and follow the listed Project Settings > Display > Window > Mode steps on macOS 15.6.1 with Godot 4.5. Compare fullscreen startup behavior against the reported version and trace the window initialization path. Done means launching the project in fullscreen no longer briefly shows a small blank window over the loading background.

Written by the indexing model from the issue text.