The editor: the Project pane, prefabs into the scene

A Project pane lists the open Unity project's assets by folder, with each one's kind and a search; it rescans on demand. Assets drag out of it: a prefab onto an object in the Hierarchy places a copy under it, onto empty space at the end of the scene, onto the Scene view where the pointer meets the ground (or at the view's pivot); a material onto an object in the Scene view or the Hierarchy becomes its renderer's material. Double-clicking a prefab places it too. The Scene view outlines itself while something is dragged over it.
A placed prefab is a copy of its objects (the runtime's {"editor_prefab"} writes the built prefab as editor snapshots, as unity_export writes scenes), with fresh ids, one undo step, selected; it isn't linked to the prefab (as Unity's "unpacked" instances), so saving writes it as plain objects.
Tests: the Coin prefab placed at a point with its components, one undo step; dragged with pointer events from the Project pane onto the Hierarchy's Ground, it lands under Ground.
Not yet: placed prefabs as linked prefab instances (overrides kept against the prefab); handles and grid drawn over objects.