In OMEdit, it is not possible to create a connection to an interface component (such as RealInput, RealOutput, PositivePin, Mechanics.Rotational.Flange_a etc)
It seems as if the component is not seen as a valid start or end for a connection, and happens for all interface components in all domains.

To reproduce this:
Create a new model in OMEdit,
add a Blocks.Interfaces.RealInput component
add a Blocks.Math.Gain component (or anything else that has a real input)
Try to connect the RealInput to the Gain.u input  - This is not possible

Also, where an existing model is loaded, which has a connection to/from an interface component, the connection is not shown in OMEdit's icon layer.