Windows 11's taskbar added a new child window that overrides the older non animated, non centered tasks list child container. I was playing with it, and I could insert my application inside the new taskbar window, just exactly where the old task list was (now hidden in Win11), but mine still shows as background, yaaay.
So (as you can see in the picture above), this can be updated when shell messages are detected, like adding or removing windows from taskbar, and adjust its size accordingly.
However, since it is a hacky method, and even if it works, it shows some annoying glitches on some circumstances, but I guess they're few. Hopefully, they will be no problem for many of us who likes to have the new Windows 11's taskbar skinned.
And about Windows 10 taskbar, I tried something similar, but as it still uses the older task list window, it can't be painted behind them, I tried, and it worked for a moment, then it is repainted and everything (skin) is erased, or sometimes, the area where the icons are shown, cutting the skinned background. So in Windows 10, I guess I will keep my application just behind the taskbar's window, that's not really beautiful, since sometimes other applications will interfere (putting their forms between the taskbar and my application window), and even worse, if taskbar is shown when fullscreen windowed applications are in foreground (like videos, etc. non DirectX games), the skin won't be shown, something that just works in my Windows 11's approach.
Yesterday Windows 11 was officially launched for general availability, well for PCs compliant to its requirements.
After a long time I logged back in here, and saw a previous post which showed an experiment with Windows 10's start menu, unfortunatelly there were many changes to how they handled its windows (aka start menu container), but it seems they just cropped its clickable area and left a child (modern acrylic) window which can't (at least by me) be modified.
So, reviving my old program Windows 8 Start Screen Customizer, it just came to my mind that why not use that on Windows 11's Start Menu, at least as its background (animated).
And here is how it looks: