Currently, when you drag a Resource from the Resources Panel from one Dictionary to another, the 'hover state' whilst dragging doesn't offer any feedback that:
a) The move is allowed
b) That the destination Dictionary is being correctly hovered

See attached video.

In the simplest method possible, we should at least consider 'highlighting' the backer of the Dictionary the drag is being hovered over, to make it clear that it is hovered.
Alternately, we could replicate the exact same behaviour and presentation we use in the Navigator when dragging one Element into a valid parent Element. (in this case, the Destination Element gets a blue outline)

EDIT/NOTE: Just noticed the same issue occurs in the Assets Panel, moving XAMLs between folders. The solution to Resources in this ticket should be replicated identically there too.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) From the Assets Panel, create a Resource Dictionary.

3) From the Resources Panel, merge the Dictionary created in Step 2), into the GlobalResources Dictionary.

4) Inside of the custom Dictionary, create a Resource of any kind.

5) Select the custom Dictionary, and click-and-drag the Resource created in Step 4) into the GlobalResources Dictionary.

6) Select the GlobalResources Dictionary, and click-and-drag the Resource back into the custom Dictionary.

Observe how when dragging and hovering over the destination custom Dictionary, there are no indications on the dictionary itself, nor in a tooltip, or any other state, that you are correctly hovered over a valid target.