DropDownButton or ComboBox with Datagrid as itemtemplate

On different places i use the DropDownButton to hide lots of information when it is not needed. When a user whant's to adjust a large collection they can click on it and do there thing. Now i whant something differend. The DropDownButton or ComboBox use a Stackpanel in there Popup to display the ItemSource Items. I want to replace this by using a full Grid, meaning heaving the headers AND rowitems. This because i use the MahApps Datagrid style which is exactly to my need.

For inspiration:
http://www.nullskull.com/a/1359/wpf-datagrid-as-combobox-dropdown.aspx
https://stackoverflow.com/questions/36615863/combobox-with-datagrid-popup
https://stackoverflow.com/questions/8744710/how-to-create-a-combobox-with-a-datagrid-dropdown-in-wpf

Start by reviewing the DropDownButton and ComboBox popup templates, then compare the linked WPF DataGrid dropdown examples. Define how a Grid or DataGrid should display headers and row items inside the popup, and consider how this interacts with the existing MahApps styling. Done means the requested dropdown behavior is implemented with the intended item presentation.

Written by the indexing model from the issue text.