The problem is that the Shortcut Panel is simply not showing.
In my installer XML, I have:
<natives>
<native type="izpack" name="ShellLink.dll"/>
<native type="izpack" name="ShellLink_x64.dll"/>
</natives>
<resources>
<res id="default_shortcut_spec.xml" src="shortcutSpec.xml"/>
</resources>
<panels>
<panel classname="CheckedHelloPanel"/>
<panel classname="InstallationGroupPanel"/>
<panel classname="TreePacksPanel"/>
<panel classname="ShortcutPanel"/>
<panel classname="TargetPanel"/>
<panel classname="InstallPanel"/>
<panel classname="SimpleFinishPanel"/>
</panels>
There are no compilation or runtime errors in the console.