Drag and Drop No Longer Working for Shader Properties

At some point during the creation of my scene, drag and drop stopped working on all the materials of objects in the scene. When I view the material component of a game object in the inspector (one that has a mesh renderer), the name of the material and shader show up, and I can select other shaders for the material, but underneath that panel I no longer have any drop targets to drag textures to, or any other shader properties for the selected shader.

Why did this happen, and what can I do about it? I’m using version 3.5.2f2.

Thank you.

I stumbled across the answer myself, so I’ll post it here for future reference. In the Inspector, clicking the material panel in the Mesh Renderer component toggles the texture drop targets and other shader properties on and off.