Inline plan editing of custom work item attributes

I've successfully created a few custom Work Item attributes and I can see them in some plan views.

I want to make those attributes editable inline right in the plan when using the Eclipse client.

However, it seems that only attributes of type Enumeration support this.  When you mouse over those attributes in the plan, you can click to get a dropdown where you choose the appropriate enumeration value right in the plan.

But if the custom attribute is a String or a Contributor type, then it seems impossible to make it editable inline to me.  The whole thing is actually quite confusing, since I don't even really know which type of editor I am dealing with.  The documentation seems extraordinarily brief and very ambiguous.  Is this the "Inline Work Item Editor" or the "Plan Editor Preview", or what?

Moreover, I don't see how those Editor Presentations have anything to do with the Plan View whatsoever.  For example, if you define an Editor Presentation for a given work item's "Plan Editor Preview", what does this have to do with the actual plan view.  When you are looking at a plan view, you can edit it and change the "View Layout", specifying which attributes are shown and in what order.  What does this have to do with the corresponding Editor Presentation that also defines a set of attributes in a specific order, and seems to be totally independent.  There must be something fundamental here that I am missing.

But I really just want to make custom work item attributes of types other than Enumeration editable inline right inside of a Plan without going to any other screen/window.

Thanks