Thank you for reporting this issue.
After thorough investigation and multiple fix attempts over the past several months, we've determined that this is caused by a synchronization gap between the DX12 SwapChain, the Editor's window event signaling, and the Windows OS windowing system. When editor panels are resized, there is a brief window where the OS has already resized the window but the DX12 SwapChain has not yet been updated, resulting in the dark areas you observed.
Workaround: This issue is specific to the DX12 graphics API in the Windows Editor. Switching to DX11 (under Project Settings -> Player -> Other Settings -> Graphics API) will eliminate the artifact. The issue does not affect built players or other graphics backends (Vulkan, OpenGLES3).
Fixing this properly would require re-architecting how these three systems coordinate, which is beyond the scope of this bug fix. We do have ongoing work related to DX12 swapchain handling that may improve this situation in the future.
Today we will be closing this case. Thank you again for taking the time to report this issue, and please let us know if there is anything else that changes the impact or severity of this issue.