When I RMB-click on any object in my project, it no longer gets a yellow profile that indicates that it is selected. However, in the Scene panel on the right, it does appear in yellow. I can also perform transformations (move, rotate, scale) on the "selected" object.
However, when I go into Edit Mode (see picture), the view of the "selected" object doesn't change, whereas normally it should change to either a vertex, edge or face view. The Editing interface does appear, but since there are no vertices, edges or faces to select, it's useless.
Did I inadvertently lock or deactivate a functionality in Blender?