Notebook cell drag and drop interferes with editor tab drag and drop

[JacksonKearl]

When dragging a notebook editor tab onto itself for splitting, it now triggers drag hover events in the editor and it it difficult to trigger the editor split event.

[roblourens]

Showing the dragover ui for non-cell dragging was fixed, but I still can't trigger the editor split overlay without going to the tab bar.

[roblourens]

This is related to moving the notebook editor out of the main grid view. We add drag listeners to the .content element, but they are never triggered when dragging a file/editor into that view because the notebook is covering the whole area. We need a way to add the same listeners to an editor which is outside of the main flow, or have the editor forward those events to the EditorDropTarget in some other way.

[rebornix]

As we discussed offline, we can take a look at how Webview Editor does it.