If in Delphi 2010 or XE Application.MainFormOnTaskbar is set to true then all secondary forms are always in front of the main window. It does not matter what the Popupmode or PopupParent properties are set to. However I have secondary windows that I want to be able to show behind the the main form.
If I set MainFormOnTaskbar to false it works, but then the Windows 7 features are broken (Alt-tab, Windows bar icon, etc).
How can I keep the Windows 7 features working while still allowing secondary forms to hide behind the main form?
Basically you can't. The whole point of MainFormOnTaskBar is to have Vista compatibility. If you don't set it, compatibility is gone.., if you set it, z-order is done. The following excerpt is from D2007's readme:
The property controls how Window's TaskBar buttons are handled by VCL. This property can be applied to older applications, but it affects the Z-order of your MainForm, so you should ensure that you have no dependencies on the old behavior.
But see this QC report, which describes the exact same problem (and closed as AsDesigned). The report states a workaround involving overriding CreateParams of a form to set the WndParent to '0'. It also describes a few problems which this workaround causes and a possible workaround for those problems too. Beware, it wouldn't be easy/possible to find and workaround all complications. See Steve Trefethen's article to have a feeling of what could be involved.