I have a form that pops out from under the taskbar. So far so good. But in the case of a transparent taskbar, the Form can’t be seen first under the taskbar and only then on top of eveything else and I don’t like this effect.

Can I somehow disable the transparency of the Windows Taskbar for a few milliseconds? Or maybe some other ideas?
I agree with @GeraldVersluis: Have the popup sit atop the taskbar. You can do this (eg, get the lower edge of your screen area) by interrogating the Screen object.
Screen.PrimaryScreen.WorkingArea