Yellow selection outline not appearing in solid 3D view I am sure I broke something: The yellow selection outline appears in the 3D viewport in material and texture modes, but not in solid any more:



... and this is because...?

The behavior persists if I append the object into a new Blender file. 

The behavior persists if I replace the existing material with a single diffuse shader node; thanks to @Leander for suggestion, however.

The behavior does not occut if the object's material is assigned to default cube. @Leander
Blender doesn't show the outline for transparent objects in the viewport.

Remove transparency in your material to make the outline visible again.