If you just stumbled across this post and don't know what it's talking about, there's an option to have objects (edges, faces, lines, etc) highlighted when your cursor hovers on them. It's a tremendous help with knowing what you're about to select before you actually click on it. I can't (or at least don't want to) imagine using the software without it. To turn it on go to Tools > Options > System Options > Display/Selection and check the box for “Dynamic highlight from graphics view”. If you had this turned on, and it turned itself off, it’s almost certainly your 3d mouse. The 3dconnexion software temporarily turns off dynamic highlighting while the 3d mouse is manipulating a model to prevent multiple edges and faces being highlighted as they move across your cursor. It's supposed to turn it back on when you release the 3d mouse, and it usually does, but not always. A few years ago it might happen to me two or three times in a day, then not again for weeks or months, but it's definitely gotten better in recent years.