Altium custom shield as component, allow other component to be inside
Note: This was written for Altium 19.0.12 (latest at time of post). It should also work for previous versions, although there may be some visual changes.

This is caused by a component clearance constraint violation.



You can add another rule that applies to the shield footprint with the minimum height set to 0.

Open the PCB Rules and Constraints Editor under Design->Rules, and navigate the tree on the left of the new dialog window to Design Rules->Placement->Component Placement. Right click on Component Clearance and select "New Rule"


Configure the new rule so it applies when the first component matches a component with the designator for the shield footprint, and where the second object matches any component. Set the vertical clearance mode to "Specified" and set the minimum horizontal and vertical clearance to 0.


Apply the changes before closing the window. Check that your new rule has a higher priority than the original clearance rule by clicking on the Component Clearance section in the same PCB Rules and Constraints Editor window that you've been working in (visible in the second picture).