Create a virtual field which is linked to the user defined portlet page.
Then this portlet page shows as pop up but in fact the window does not have the behavior.
Step to recreate.
1.Create a new link on task object which linked to a portlet page.
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2. Show the Gantt and create a new virtual field which linked to the above link and check for "Open as Pop-up.
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3. The new virtual field is shown on Gantt List.
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4. When click the icon then I can see that the Gantt window moves to the link page without Pop-up.
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If the virtual filed links to the existing link such as task properties then I can see that the Pop-up page opens.