WiX Burn: MsiPackage internal UI display but behind the custom WPF UI - user-interface

Question

I am creating a custom bootstrapper which has a WPF UI.
First: the WPF user interface will display.
Second: when I click the install button, the MsiPackage will execute, the internal UI will display because I set DisplayInternalUI to yes
Everything is OK, but it has a problem. The internal UI displays behind the custom WPF UI.
How do I set the internal UI to display at the top of window?
I tried to minimize the WPF UI when I click the Install button. Yes, the WPF UI will minimized, and seconds later the internal UI also... It shows with the minimized window state too.
My bundle.wxs:
<BootstrapperApplicationRef Id="ManagedBootstrapperApplicationHost">
<PayloadGroupRef Id="InstallerPayload"/>
</BootstrapperApplicationRef>
<Chain>
<PackageGroupRef Id="NetFx40Full"/>
<PackageGroupRef Id="InstallerPackages"></PackageGroupRef>
</Chain>
</Bundle>
<Fragment>
<PayloadGroup Id="InstallerPayload">
<Payload SourceFile="..\CustomBootstrapper\bin\release\MahApps.Metro.dll"/>
<Payload SourceFile="..\CustomBootstrapper\bin\release\System.Windows.Interactivity.dll"/>
<Payload SourceFile ="..\CustomBootstrapper\bin\release\CustomBootstrapper.dll"/>
<Payload SourceFile="..\CustomBootstrapper\BootstrapperCore.config" />
<Payload SourceFile="C:\Program Files (x86)\WiX Toolset v3.7\SDK\Microsoft.Deployment.WindowsInstaller.dll"/>
</PayloadGroup>
</Fragment>
<Fragment>
<PackageGroup Id="InstallerPackages">
<!-- <MsiPackage Id="CRMInstall"
Vital="yes"
Name="Packages\SetupProject1.msi"
SourceFile="Packages\SetupProject1.msi"
DisplayInternalUI="yes"> -->
<MsiPackage Id="CRMInstall"
Vital="yes"
Name="Packages\StandardSetup.msi"
SourceFile="..\StandardSetup\bin\Release\StandardSetup.msi"
DisplayInternalUI="yes">
</MsiPackage>
</PackageGroup>
</Fragment>
..
..

Answer 1

When you call Engine.Apply() make sure you pass the handle to the WPF Window that is being displayed. It should go something like:
var mainWindowHandle= new WindowInteropHelper(mainWindow).EnsureHandle();
Engine.Apply(mainWindowHandle);
Where mainWindow is class that inherits from the WPF Window class.