Tested versions

Broken in 4.4.stable
Works fine in 4.3.stable

System information

Godot v4.4.stable - Windows 10 (build 19045) - Multi-window, 2 monitors - Direct3D 12 (Forward+) - dedicated NVIDIA GeForce RTX 3060 Ti (NVIDIA; 32.0.15.7270) - Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz (8 threads)

Issue description

When the app with D3D12 starts in windowed mode, it has decorations, but content in transparent until it shows the scene. It used to be plain black until it shows something. Same goes for the editor -- editor starts in transparent, when opening subwindows, they first appear transparent too.

This wasn't the case in 4.3.stable.

Steps to reproduce

1. Enable D3D12
2. Run app in windowed mode
3. See that it start with transparent background

Minimal reproduction project (MRP)

window-transparent-d3d12.zip