Window size not constant in borderless mode

### Tested versions

reproducible
- v3.6.beta.custom_build [02c02403e]
- v3.5.3.stable.official [6c814135b]

fixed in
- v4.3.dev5.official [c9c17d6ca]

### System information

Ubuntu 22.04.4 LTS, NVIDIA 535.161.07

### Issue description

Noticed not constant window size in **borderless mode** in **3.x**. But looks like this is **fixed in master**

```
sample output
(1024, 637)
(1024, 600)
(1024, 600)
(1024, 600)
(1024, 637)
(1024, 600)
```

### Steps to reproduce

1. run attachment
2. press any key
3. repeat until you get different results

or

1. make new project
2. enable borderless mode in project settings
3. add following script to main node

```gdscript
extends Node

func _ready() -> void:
print(OS.window_size)

func _input(event: InputEvent) -> void:
if event is InputEventKey:
get_tree().quit()

```

### Minimal reproduction project (MRP)

[New Game Project.zip](https://github.com/godotengine/godot/files/14783548/New.Game.Project.zip)