Project manager window resizes then moves during startup

### Tested versions

- Reproducible in 4.3.beta (a0bbd398ee72f8c8e49d50d2e47e9fd36aa9b337)
- Reproducible in 4.2.2-stable
- Partially reproducible (one move, no resize) in 4.2.1-stable and earlier

### System information

Fedora Linux 40 (KDE Plasma) - Wayland - Vulkan (Forward+) - dedicated AMD Radeon RX 7600M XT (RADV NAVI33) () - AMD Ryzen 7 7840HS w/ Radeon 780M Graphics (16 Threads)

### Issue description

I noticed that at least on my Linux system on KDE, when starting the project manager, the following happens:

- The window is shown with the splash screen centered and window size 1154x677
- The window is resized and the splash screen is now off center, with window size 1026x629
- The window is moved towards the top left when showing the project manager, still with window size 1026x629

This is how it looks like in action:

https://github.com/godotengine/godot/assets/4701338/4faeeb92-a5dc-4edc-8282-ba9ad7d3e9a3

Screenshots to compare sizes and positions:


### Steps to reproduce

- Start the project manager on Linux / KDE

### Minimal reproduction project (MRP)

n/a